Interestingly, baler belts go through a few different stages of 'stretch' in their life. For the first couple of years they'll actually shrink a little (unless stored with tension on them). Then they'll start to stretch over time. Yours are likely at this stage, where they're longer than the factory length. It's not going to hurt anything and only help to get them cut back to factory length and re-laced. The only thing that will cause trouble is if they're inconsistent lengths. You can actually go up to 4 or 5" under factory length and still be fine - the only thing it will affect is not being able to make quite as large a diameter. On our JD 435 the belts are now 4" under factory spec., so we can't go quite to 6' diameter. Doesn't matter for us as we never bale that large anyway.

If yours are overlength now, I wouldn't want to have them re-laced to that extra-long length: They'll have to cut 1" or so off the ends to get good material to lace to, so to get to the current overlength they'd need to splice additional pieces into the belt. Some folks do that, but it's usually only done as an interim measure to fix a broken belt and get through a season. It tends to cause grief tracking them, and is harder to ensure they're all a consistent length.

You definitely want someone that makes them a consistent length, but cutting belts and installing clipper lacing is pretty straight forward. I suspect your Deere dealership (or any other dealer) does it all the time, so they'd probably be as good as anywhere. Things to make sure of are that the belts are cut square, the trailing edge has tapered/chamfered corners, and the leading lace has the outer-most rings on both sides. I think with the 3-ply diamond pattern Deere belts you might have to skive off the top diamond cover, but I could be remembering that wrong. The Deere dealer would know.

I always use the 'smart locks' to retain the wire on clipper lacing because I like the idea of tying the last few hooks together on either side. I've had good luck with them, but I have also heard others complain about them. Ultimately I don't think it really matters. As long as they're tracking ok, the regular method of just tucking the wire back under the hooks works fine. When you go to tuck the wire back under, sometimes there isn't enough room to get it under there. An old trick is to heat a nail red hot and jam it in there so it melts/vulcanizes a path for it.