Opted out of the conservative approach I see...I don't think you're too late to repot beech, by the look of the buds. Just be extremely careful overpotting trees; particularly in bonsai containers. Unless your soil is very porous, it will very easily lead to root rot.
Hard to tell from the photos, but it looks like you have one more decision to make on a large branch toward the apex. A back-left branch is about the same diameter as the main trunk line. I'd remove that sooner than later. They take FOREVER to heal; fortunately it's to the back.
Aside from that, it is a great tree, and J. Beech are special...we should be so lucky to have a few good ones in the states, and yours has the base and trunk line to become a great bonsai. Study up on training them, they are a different animal!