Good morning, I have a a well established Azalea with a one and a half inch thick trunk vibrant foliage with slowing new shoot growth. it is in a 12 by 12 by 6 wooden grow box and has been for two years. The soil is starting to breakdown and the box is starting to break up a bit. Question is could I repot it now before the temp gets cold. Right now in Atlanta the forecast is highs mid eighties, lows in the low sixties.

here are some pics they show some progression

It's possible to repot in autumn however advised is to go for early spring.
Only if you have a place were you can protect them in the winter, repotting in autumn is fine.
It shall need more protection then if you were to do it in spring.

thank you so much for your response, I don't plan on removing much if any roots, and really want to introduce some fresh soil and move it into a Bonsai pot. If you dont mind what is your opinion about the tree especially the one with all the foliage.

My opinion is that for bonsai purpose the one with the most leaves is a species with too large leaves.
I think leave reduction can be difficult on that on.
Nevertheless the both have some potention to become nice looking trees, but keep in mind it can take many years before they are.