It's not too late but I wouldn't leave it much longer. Really needs to be done before tree has leafed out.

Done now it will certainly affect the new shoot growth. Why are you interested in pruning the roots?

I doubt that will be a concern - I have root pruned many JM's rather later into the season than I would have preferred and they have all done just fine. I have also on occasion root pruned them in the middle of the growing season and they withstood that also without issue. Just don't get too carried away!

Root pruning is a common practice with containerized Japanese maples (and any other trees) and is something that is done every three years or so unless up-potting.

There is no way that cutting the roots right now will not affect top development. I didn't say that the specimen was likely to be killed, just affected. As in dwarfing etc. This is why bonsai are root pruned as an annual maintenance procedure - cutting roots has a dwarfing effect on top growth. What to do now - or later - depends on what O.P. is trying to accomplish.

There wouldn't be much of a reason to root prune unless one was growing the JM in a container. And the visible impact of root pruning now (or even later) is negligible - growing any tree in a container long term is a dwarfing process, whether you root prune routinely or not.

This maple has been in the same container and soil for about 3-4 years. Last year it was not doing well as previous year.

Wanted to change soil and trim the roots. I could just leave the roots as is and just change the soil, no?

If it has been in that pot for 3-4 years, once you unpot it you will see something will need to be done to the roots. Either pruning or getting a bigger container...or both.

tj