Hi. I'm really new to this and am looking for advice.

I have just got new Japanese Maple tree from a nursery. I would like to start training it as bonsai but for that I suppose I need to repot it to new bonsai soil - currently it sits in regular thick soil from the nursery since it was normally grown to be simply planted in a garden.
It's late fall and normally I'd repot it only in early spring. But since it's sitting in a wrong pot and wrong soil, should I repot it?
If yes, should I cut roots or keep it as is? How soon can I start wiring it?

Thank you all in advance

I'd wait till mid spring to report

If the tree is healthy there is not reason to repot in the fall. remember, trees store energy in their roots over the winter... if you cut them, or disturb them, you may rob the tree of energy it needs to grow. Japanese maple has a very small window when they can be repotted. The nursery maple that I have is still partially in it's potting soil after two years, as I have it in a grow pot. I am probably going to repot in the spring, into a smaller pot, or maybe into the same one, depending on the condition of the roots.
have a look at this:
www.bonsai4me.com/SpeciesGuide/AcerPalmatum%20page2.html
or this:
www.bonsaiempire.com/tree-species/maple

the intermediate course on bonsai empire also has a good deal of information on maples.
wiring and pruning can be done in winter/late fall... when there is less sap flow, and you can get at the branches easier then when the have lots of leaves.

That's what I thought, better repot in spring.
My biggest concern with this right now is that the soil the trees have right now is very dense and does not drain quickly, it keeps water for a very long time.
Can it cause root rot?
How often should I water the plant, since usual watering advice won't work because of different soil?

Think it would do fine till spring and water when soil appear to dry on top