Now!! Much less of a disruption to the tree if you do so while it is still dormant.

Thanks. Would this include conifers as well? I have several small conifers that I would like to put in larger containers.

Yup - always a good idea to do any significant root disturbance while the plants are dormant. Repotting can occur at just about any time, if not messing with the roots substantially.

Best time to repot almost all maples is at first signs of budswell, and you can repot up until about the time that leaves just start emerging from buds.

Repot larch before buds open. Pines (other than mugo) + yews + junipers are best repotted 2-3 weeks after your deciduous trees are repotted - they warmer temps at repotting time. I repot conifers in May. I'd guess, for you, early to mid-April would be about right.

Pot up (as opposed to repotting) almost any time, but you're less likely to be bedeviled by over-potting if you pot up when your plant is actively growing. If potting up is part of your care regimen, you would do better to pot up BEFORE the root/soil mass gets congested to the point where it can be lifted from the pot intact. Once that occurs at any stage in the plant's development, you'll need to physically get into the root mass and correct root issues to avoid limitations associated with root congestion.

Al