From the upgrader allow to choose to upgrade any themes. By default select all themes, but allow unselecting and changing the from/to versions.

Working on this in a new Github project:
https://github.com/ocproducts/convertr

Chris, it would help if you can mention what is already implemented and what needs to be done for Convertr. There are a lot of TODOs in the Convertr\apply|generate\opcode\file namespaces, maybe I can start there?

Also would be great if in the issue description you can put a strike-through on the features we are no longer gonna implement.

Moving to Convertr project, as separate issues

I'm considering us dropping this project, I haven't decided yet. It is pending my review.