Over here in the UK we're the opposite way around, 27MHz/11Meters FM is our legal band, AM is illegal (although I do own some old Cobra AM rigs and have been known to earwig whatever skip I can pull in on a nice evening, just don't key up lol)

Does the rig you're looking at have a little "27/81" logo on the faceplate somewhere? If so then it'll be designed for use on the UK band also.

From experience, FM is definitely very much clearer and higher fidelity BUT very much less range-per-watt compared to AM, with zero chance of any "skip" (where you can hear folk clear across the country on a clear evening).

One other advantage to running FM there, if you're communicating between a local network where you can all be on FM then you would be far less likely to get randomers coming in over the top of who you're actually calling (I gather channel 6 over there is quite ridiculous now & even 19 gets congested with folk who don't know how to take it up or down a few after making contact.

Something else I read led me to believe that there are devices one can add on a straight AM radio to provide switchable FM ability; of course, as such things go, one could probably buy a new AM/FM radio for about the same money.   

Not sure on the add-on convertors, I'd imagine it's possible but like you say they're probably not cheap.

Did I hear a request?

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Having both AM (the historical 'standard' for US Citizen's Band) and FM modes, does NOT constitute having '80 channels'.  It means you've got two different modulation types fighting over the same 'channel'... interfering with eachother.

Let's do some basic theory here.  A radio transmitter is a device which has, at it's simplest configuration, an oscillator.   Like a BELL... it rings.  It's ringing at a frequency much higher than what you can hear, but it's ringing... and it's not generating mechanical air pressure, it's generating a magnetic field that is alternating.  Because of what it does, it 'disrupts' the magnetic environment around us.  Most radio-frequency activity occurs between 540 Kilohertz (the bottom end of the AM broadcast band), and 5.0 Gigahertz (your cellphone's higherp-end Wi-Fi connection).  Frequencies in the lower ranges tend to 'bounce' their way around the globe, while frequencies higher up tend to be line-of-sight.

IF you take a coil of wire, and connect a circuit that can gather that disruption, you can hear the PRESENCE of a transmission.

IF one end turns that oscillator on and off... and someone far away can HEAR that oscillator turn on and off, you can 'key' out a signal using the off-and-on... and you have radio telegraphy.

Now let's say you increase the pitch of the 'note'... go up to 4.3 Terahertz... that's the visible light spectrum.  Turn a light bulb on and off, and you're transmitting a radio signal by telegraphy... because light is electromagnetic, and our eyes CAN DETECT radio-frequency energy in the 4.3 Terahertz range!!!

So turning a light on and off, is technically a radio transmission and reception method, and by turning it on and off, you place intelligability in the transmission.  Two boy scouts with flashlights sending morse-code.

Now here's where 'modulation' comes in.

Continuous Wave (CW) is simply the circumstance of a signal being off or on.  You can call it 'Morse Code', but technically, it's a Continuous Wave mode, that USES a series of short and long on-times, separated by off times, to yield a message.   Radio guys refer to this as "CW".

AMPLITUDE Modulation, is when the INTENSITY of the transmit energy is varied, to yield an intelligable signal... it's basically a circumstance where, instead of turning the light on and off, you turn it on, but you VARY the brightness (like, with a microphone) to pass some message from one side to the other.

Frequency modulation, is changing the 'note' of the bell... or in the case of light, changing the COLOR of the light... and the message is sent through the change of color.

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The amount of 'power' required to carry a signal, is highest using FM.  It 'sounds' quieter, because the presence of static has no impact on the change of frequency that the reciever is looking for... but in order for information to be carried through, the modulation must swing the pitch up and down, such that any 'channel' above and below will be encroached upon... resulting in interfereance.

AM requires significantly less power, and that power is focused on just ONE frequency, so you can have 'channels' much closer together than in FM.

Then there's Single Sideband Suppressed Carrier, aka "SSB".  SSB is an AM signal, but where the actual carrier frequency, and ONE HALF of the modulation waveform domain has been removed.  The receiving station 'adds' the carrier and other sideband, to reassemble the intelligable signal.  This means that all the transmitter's power is thus focused on the modulation envelope that remains... all the transmitter's power goes into the intelligable signal.  As a result, SSB and CW carry much farther because of their higher efficiency... and they require less 'bandwidth' to do what they do.

Finally, there's CW... which is the most effective at reaching out the farthest with the least amount of power.

FM was approved by the FCC for use in the US, but it's a very foolish idea... very few 'cb operators' have a clue about how RF energy works... and in the modern 'free-for-all'' circumstance means everybody will be doing everything everywhere, walking on everybody else.

When a radio is equipped with circuitry for upper and/or lower sideband, it will, by design, have a much better receiver... it will hear better, EVEN if you're using AM.  It USED to be considerably more expensive, but from a manufacturing standpoint, adding the componentry for sideband operation is insignificant... to the point that if you open up and look, you just might find that the circuit board in your AM-only Cobra might have a section set aside for all those components... they might actually BE THERE, and just not 'enabled' for operation.  Building things this way reduces the quantity of components a manufacturer must build and support... it's like cars fitted up with all the wiring for electrically-heated mirrors... but you didn't 'buy' the package, so the switches were replaced by 'blank' panels.

I didn't put hard numbers in my description above, and in retrospect I SHOULD have, and as I was driving along today, I realized the 'easy' example was right in front of me.

Turn on your AM broadcast band, and you'll notice the numbers are 1110, 1120, 1130, 1140...  those are in Kilohertz... 1110 is 1.110 Megahertz... 1420 is 1.420 Megahertz.  AM Broadcast stations are 'assigned' frequencies in the AM Broadcast Band at the 10,20,30... not the 15, 25, 35, etc., because the broadcast SIGNAL extends from 5 kilohertz below, to 5kilohertz above.  IF they put a station on the 15 mark, or the 45 mark, etc., it would visciously interfere with the station 5khz above, and 5khz below.

IN FACT... when they issue licenses, they look at the  location, and power level of the OTHER stations on either side of a proposed new license, and set up spacing so that a station on say... 730, isn't placed on the air nearby a station on 740... because anyone listening to either, will get terrible quality from the adjacent frequency's signal.

Now, with all that in perspective, the AM Broadcast signal is, by this explanation, about 10khz wide.  730 would occupy from 725 to 735.  740 would occupy 735 to 745.  Seems kinda wide, yes?  Well, if you narrow it up (and you can), the result is bad sound quality... like everybody's talking with pencils shoved up their noses, and a rag in their mouth.  Remeber 'sideband'?  Well, the space from the 'center frequency' to it's outer edges, is the side band.  For 740, the 'lower' sideband occurs from 740 to 735.  The UPPER sideband occurs from 740 to 745.  If you add some fancy circuitry, you could chop off just one side (upper or lower) and suppress the carrier (which is why SSB is actually referred to as Single Sideband Suppressed Carrier) and get all the intelligability, with significantly LESS transmit power... because the RECEIVING set replaces the missing carrier, and converts the single sideband, into the equivalent of an AM signal.

Now let's switch to FM Broadcast.  Notice how the channels start around 87, and go up to 108?  That station at 101.3 is 101.3 MEGAhertz... over 100 times HIGHER than your 1100 Kilohertz AM Broadcast station.  Notice the FM Broadcast channels are in odd decimal steps too...   101.3, 101.5, 101.7... never 101.4 or 101.6.  What this means, is that every FM Broadcast CHANNEL is 0.2 Megahertz WIDE.

0.2MHz is the same as 200 Kilohertz.

The US AM Broadcast channel is 10 Kilohertz... there's 110 'channels' from 540khz to 1660khz.

Notice a difference here?  Yeah, FM is SIGNIFICANTLY wider... and the reason is simple-  The FM signal is changing frequency up and down... it is naturally wider.

IF one were to convert the AM broadcast band to FM, using the same concept, instead of 110 AM broadcast channels... You'd have just 5. 

How's this compare to CB?

In CB, you have 40 channels spread from 26.965Mhz to 27.405Mhz, they're on 10Khz spacing, just like AM broadcast.

Go to FM, and your channel spacing results in... FOUR channels.  Just four.

And when You're using FM, you're walking all over the users of TEN regular AM channels.

And this is why people that understand radio technology say that arbitrarily opening up CB band to 'FM' is a foolish thing to do.  The FM mode is much better suited to higher frequency operations (above 130Mhz), and that's precisely why MURS, GMRS, and FRS exists... with the added bonus that those higher frequencies are less prone to atmospheric noise, are SIGNIFICANTLY more efficient to utilize with shorter GAIN-type antennas necessary for vehicular operation... and they're allowed significantly more transmit power to boot.  GMRS and FRS allows repeaters, too, and there's quite a movement to the growth of public GMRS repeaters.

As usual, Dave hits it out of the park

Back in the mid-late 1970's I was an Avionics Tech (COMNAV) in the Marines stationed on a helicopter base in Tustin, California. The Marines were still flying outdated leftover junk from the Vietnam era, some of which had to have been designed in the late 1950's-early 1960's. Some of the equipment was transistorized, some was sorta hybrid with both vacuum tubes and transistors, and some was pure vacuum tube stuff.

The helicopters had UHF radios (ARC-51) for air-to-air and air-to-tower communication, FM radios for talking to the ground troops, and HF (ARC-94) radios for long range stuff.

I was primarily a RADALT (Radar Altimeter) tech, but I cross-trained on a number of other black boxes as well. On of those was the ARC-94 HF unit. The ARC-94 was probably the pinnacle of vacuum tube airborne radios; a veritable monster weighing in around 40 lbs (I forget the exact specs). It had a chassis that contained a complicated mess of mechanical gear trains that were servo-controlled to facilitate changing channels by mechanically rotating all of the various modules that plugged into the main chassis. They probably worked fine when they were brand new, but the wear and tear of constant vibrations, etc. made it difficult to keep those ganged tuning capacitors working properly across the full range of its 30,000+ possible channels.

The ARC-94 had AM plus upper and lower SSB capabilities; minimum power on AM was 125 Watts RMS and Sideband had to be 400 Watts RMS minimum. At night time we would routinely contact Japan for radio checks. I can still remember keying the microphone "Yakota, Yakota, this is Hotel Michael Sierra One Six on eight niner eight niner requesting upper HF radio check, how copy?" A few seconds later we would hear the reply "Hotel Michael Sierra One Six this is Yakota on eight niner eight niner, copy fair and readable". Ah, those were the days!