If at all possible, you would be far better waiting until the autumn time before repotting your Acer Tree as the tree will then be dormant.    If you cannot wait until autumn time I would suggest that you lift your tree as soon as possible before it is actively growing.

I'd do it now if it needs it - I used to always do mine in early spring.

I have quite a few in pots and always re pot as they come into growth (usually march) so you should have no problems doing so now.Where these grow naturally they get very cold but dryish winters , and so i would avoid re potting in Autumn as they wont like sitting in wet compost over winter.

id do it now but would of been better a couple of weeks ago .