When its dormant I,d say
After leaf fall in the autumn, usually november and before bud burst, usually around march.
Unless its an emergency repot of course.......

As long as it's going from a smaller pot to a slightly bigger one, i can't see a problem when you do it as long as the rootball is kept intact and it's watered well. I'd happily do it now.

Hum as we now have what passes for summer I'd wait until autumn - easier to keep the maple watered then. I know in theory Andy that so long as you keep the rootball intact you can repot at any time but I am wary!

I'm sorry to disagree, but spring time and up until now. Reason being - acers hate being in soggy compost over winter. If potted while still in first strong flush of growth the roots get going and fill the pot, so when the rains come, they have the potential to take up the excesses of water in the compost. Believe it or not, acers in pots are quite happy on the dry side if there's no drying wind over them (hence the reason for a sheltered site). If you pot up in autumn, the roots are left in a huge pile of potentially wet compost, (which also might freeze), leading to the roots dying, rotting. Then, in the spring, the top of the plant starts off, the new leaves emerge, but there's no help from underground and you get dieback, and DEATH!

Early spring then but I,d still do it whilst its dormant, less stress all round