Re-potting Hosta

If all you are doing is putting the plant in a bigger pot without disturbing the roots too much, then you may repot them as and when you choose.

I would wait til Autumn and take it out chop it up and replant the pieces in new pots.

If going down that route, I would personally do it in early spring just as it starts to shoot. In general, small plants tend to resent sitting around in large amounts of cold compost over winter

I've read that what Andrewr said is probably the best time, in early spring just as you start to see new growth coming up.