I wouldn't do much repotting now. Not really enough time before winter. You may be able to get away with it, but you may not, depending on how agggressive you are. Not much point in repotting a boxwood if you aren't planning to agressively reduce the root mass. That's best done in the spring.
I leave this and my other Korean Boxwood (buxus Microphylla 'koreana,' now called, I think, Buxus sinica) out with only four inches of mulch on the ground in the winter. Korean box variety is one the hardiest of the Buxus lot. It is hardy to Zone 4 in ground and probably near that in containers. I've not had any problems overwintering this here even without winter protection.
English boxwood (Buxus sempevirens 'Suffruticosa') is winter hardy to Zone five inground. They're not as vigorous as the Korean boxwood