Not an ideal time but it can be done. I've repotted JM's in midsummer a number of times - avoid a really hot day, make sure the plant is well watered first and complete the process as quickly as possible. As a matter of fact, I've a got a Golden Fullmoon I'm considering repotting myself :-)

Make sure you use a very textural, barky potting soil......no MiracleGro stuff. It's too fine, too moisture retentive and doesn't provide the fast drainage and proper aeration these trees require in a container.

I repot all year long as needed. No problem.
Follow Gardengal's advice and it won't be a problem for you too.
In colder and or, hotter zones, it's a different ballgame.
Mike

As long as you can keep your maple in the shade for a week or so after, June is a very good time to repot. The roots should be growing strongly and plenty of time for them to get established before winter.

I have repotted my new aka shidare momiji about 3 weeks ago or so, and infact 2 times in one week as I was given wrong advice on soil and the glazed pot I used got rather hot under the sun. in fact the last move to its current container, I had to wash away most of the soil on the roots as the soil would have been quite different from the new mix and caused problems. now its in a large terra cotta pot with good draining mix soil.

I'm pretty new to this, so I guess I would not advocate what I have done, but seems like after 2 weeks in shade the tree is back to its health and budding new leaves well.

some pictures I took yesterday and today..

Here is a link that might be useful: my new aka shidare momiji