I collected quite a few trees last fall right after leaf drop and all survived. I also repotted a quince and it is doing wonderful.I am in zone 8a
We have also collected trees in Fall (even with leaves in October) and it seems that they are healthier that trees collected before growing season starts (Spring). Surviving rate is also much better in Fall.
Usual thing is to repot in Spring, but in our warmer climates, I thing that it is possible to repot also in Fall, in that way when Spring comes, tree is much stronger, so we avoid usual slowing down after repotting.