Bonsai Heresy mentions uppotting "frequently " without defining "frequently".

In late February the four year old grandson gave me some seeds from an apple and very excitedly said I could grow an apple tree. I put them in a clay pot about 10 cm. One of them sprouted and is doing well, about 15 cm tall and lots of leaves. He asked me about the tree this weekend, so the memory is there and it is important.

For some reason I stuck a cutting from a Japanese maple in the same pot (it was right beside the tree I was trimming) and it has been growing strongly, too.

So, does "frequently" mean right now? Next fall? Next spring?

I think that for seedlings there is a benefit to growing them initially slower. That way you get shorter internodes which it good for future branches. I would therefor say, just leave them where they are and work them in spring. Certainly as this apple is kind of special: no need uprooting?