So repotting season is near, and I was wondering how much of the old soil so of you remove, when you repot maples. I think, that I was a bit too conservative last year, when I repotted and didn't have some of the growth I was hoping for. because I didn't take out enough of the old soil, and it got compacted etc..
what are your thoughts?

This would depend on the development status of your maple, and maybe the school that you adhere to I would say.

For young maples I remove all old soil (And in many cases, 90% of the soil comes out pretty much by itself). I do not not have well-developed maples.

Ok... I get the idea. I'm probably going to bare root the two I wasn't so happy with... I have a fairly established one that is not in a free draining soil at all.. basic potting soil, as far as I can tell. I want to transfer it to a more modern substrate... would you do this in one repotting, or gradually?

Hard to tell. But I normally am ruthless doing the work in one go, unless there will be no feeder roots at all.. But normally.. There are feeders near the trunk

I guess the question would be, is there any danger to a more conservative approach?

I remove almost 100% of the old soil, but when in doubt, keeping 1/3 of the old soil is always a safe amount.