Best time for transplanting Japanese maples

When is the best time to transplant Japanese maples from the ground to a pot? I have had a couple of trees planted for a couple of years now. They are planted over tiles so the roots would spread out, not down. Just wondering when would be the best time to put them in a pot. Fall or Spring? Any serious advice is always welcome and appreciated!

I transplant maples in the spring. I have had great success with doing a great deal of root work including washing so that I can arrange them at that time. I have done some far less intrusive transplants through the summer, but I don't recall doing anything in the fall. My concern with fall transplanting from the ground to a pot would be to get enough root growth to do well over the winter and come out in the spring. On the other hand, from pot to ground is apt to be less stressful, particularly if the roots are not cut back too much.

I appreciate the info and quick reply, that's what I thought but I wanted to hear it from someone else. Many thanks.

Glad it was helpful. I forgot to mention that I have also transplanted 2 - 6 year old seedlings in full leaf with bare rooting and some root rearrangement during the summer as well. I picked a cooler stretch (<80F, 30C) and kept them in partial shade. Not what you are planning to do, but I figured it might be useful to someone.