That's a good idea about piece that can push up from the bottom. It would be pretty easy to incorporate into the design.

I'm actually thinking about designing a multi stage transplanter as well.

The idea is a small pot, which has a removable bottom, (that snaps on and off) that can stack on top of another bigger pot, in which this pot will also have a removable bottom that can stack on a final 3rd pot. Almost like the idea of a Matryoshka doll except the pots stack on top of one another instead of going inside one another. If you can envision what I am planning :)

I don't think this will be easy, because of certain considerations with the root system but perhaps if it flares a certain way, it can work.

Essentially this will make it so transplants do not disturb the roots at all, and ease the process for the grower. :)