The ideal time to re-pot an Acer is Feb-Mar, just before it leafs. If it is still in bud (some varieties are slower than others) then you can do it now. If it is in leaf then I would leave it till autumn unless it is badly needing re-potted. I had to re-pot one last week as it was not looking good - the buds looked dry and when I took it out its pot it was very dry and pot bound, so I repotted and fed it to give it a good start to summer growth.