Most Bougs are going into dormancy if they haven't already..I'd wait until spring, and if need be, repot to 1-2 sizes larger pot w/well-draining soil. Bougs do not like wet feet, Warren. (Like most plants) Be careful watering this winter..Only do so when soil feels completely dry..Toni

I agree. Wait until the spring. Bougs will grow in just about any dirt ( though they flower better with some Potassium) but they do resent being moved in the winter.

When is the latest to safely repot (or plant in ground)? I just bought two large lovely baskets, but they are root-bound. Is it better to wait until the spring?

I am in Z8 and mine are still blooming, not into to dormancy yet here, they are blooming everywhere, Mine if covered do well until late Nov or early december, they are in the ground,

You can wait till spring if you want to. Bougies don't mine being potbound at all and I find that they will bloom better that way. Once you repot they will start putting out roots and not bloom as good the first year. Mine usually bloom the best in fall anyways.
I would pot up only a size or two to make sure they have excellent drainage.
~SJN