The timing is rather critical when repotting. It is best done when the tree is dormant - out of leaf - sometime in winter or before bud break. I have repotted or up-potted new maples when I've purchased them during the growing season but you do run the risk of transplanting shock so it must be done very carefully and with great attention to aftercare. In most cases, I leave them in the container they came in until a more suitable time for repotting.

If you are just up-potting or moving into a larger sized container, there really isn't any reason for root pruning. If you DO need to root prune, you want to do this also during dormancy, never when in active growth. An argument can be made for removing as much of the current potting medium as possible, depending on what it is - if the very barky mix that most JM growers favor, there is less need to remove than if they were potted in a very fine peat-based mix. And you don't really need to soak the roots before repotting - I'd just make sure the plants in their current container were well hydrated before the repot and watered well after.

My description of the roots being 'non-aggressive' means that the roots are not overtly water seeking as some tree species are nor are they at all inclined to damage inground plumbing or foundations/paving. But digging up an inground planted tree that is well established will not be an easy task. And it is difficult to contemplate digging one up and putting it into a container successfully unless a very large container!! Not something I'd necessarily recommend :-) Again, the optimum time to do any of this is when the tree is fully dormant.

Garden gal - on these same lines, I have a Ryusen that's not doing well - I have prepared Al's 5:1:1 (will use gritty mix during dormancy.). What precautions should I take when potting up.