Preface; I'm no expert on bonsai, nor on maples. BUT, I have grown lots of trees in containers.

First, you said you need to re-pot now. I would suggest only potting-up at this time. Do a root prune, change soil, and re-pot next dormant season. Also gives you a chance to begin improving root appearance for 'nebari' (surface roots, common in bonsai).

I'm guessing your tree does not have a thick enough trunk to even think about training it as bonsai yet. I'm also guessing you will have years to go.

Best advice I would have without knowing more about your tree, or about what you want: pot-up your tree, then read a bunch on bonsai techniques, and begin your work next dormant season.

Itching to get started on a bonsai right away? Go to a nursery and get a tree that already has a thick trunk to begin your work.

Good luck, and CHEERS!

Hi, it's not clear to me where your tree is growing - indoors or out, and that's going to matter a lot as to what happens next. As well, can you please post a picture somewhere (leaving a note here) so we can see the tree, which would be a big help, especially if you can get a good shot or two of a relative close-up of the leaves. You didn't say where you got the tree, but if it was your backyard (or equivalent) you may be growing e.g. a Norway or sugar maple, neither of which are much good for bonsai at all. Thirdly, the best time to work on maples is the winter, when they're dormant (if outdoors where they belong), usually in Feb. before they bud out and before sap flows. Early and mid summer are bad times, so what you might have to do if the pot's too small is get a size larger pot and 'slip pot' - get the whole thing out of its present pot in as much of one piece as possible, disturbing roots as little as possible, and just slip it into the larger one, backfilling with a gritty mix where needed. That would be temporary, but at least deal with the scrunch.

One other thing - you can trim leaves and twigs now (though a month from now would be best) to shape and clean up the foliage, but not major branches.