I'd wait until they lose their leaves and go dormant.

Spring would be safer as the roots won't grow much,if any over winter in pots and could rot sitting in wet compost over winter.

It depends whether they're root bound, really. If there's plenty of room in the pots, just leave them until the spring, but you could do it after they lose their leaves, as Steragram says, so long as you don't disturb the roots, and you water them really well. Then you can leave them in a sheltered place and give them a feed in the spring. I have loads of birch seedlings that have self-seeded in my garden, and I pot them up at any time of the year, taking lots of soil around the roots, except if it's frosty. Annie