Thanks .. i did it by reflash bootloader .. unofficial .. then official ..
The upgrade from 12 to 13 sounds pretty simple, using those steps.
The part that's still unclear to me is, is it an in-place upgrade that leaves all my apps intact, or does the system end up with a plain OS and I need to install all apps from scratch?
jjpprrrr's official documentation talks about "partition scheme has been switched", which sounds a little like data-loss. it also asks to "Format data/factory reset", and that would delete all apps, right? is that steps not required?