Best time is dormant period in winter. I have never had any problems in repotting Box at any time though - if it is badly root bound then I think it is best to repot now allow the Box to benefit from its main growth period in June-August. I just repotted mine.

Keep well watered if you do it now

I'd repot now too - and invest in some John Innes No. 2 or 3 potting compost to do it with. If you use that, you won't need to feed the plants for 4/6 weeks afterwards.

If the roots are severely matted, so you have to tear or cut them to get them to spread out, then trim them enough to remove about 1/5 to 1/4 of the leaves. That reduces the stress on the root system, so it can grow new root hairs before it dries out too much.