Well, the answer seems not to be easy…
My daily used system is using an encrypted data partition (and /e/ nougat version).
If I install the oreo version on top then there’s a bootloop.
If I install the pie version on top then it says - when booting - that data partition is corrupt and that a factory reset has to be done. I can just accept that and it reboots into recovery to do the factory reset.
After installing /e/ nougat version without encrypted data I’ve been able to install pie version on top (without wiping anything) and at least a handful of installed test apps seem to work fine as before (with all configuration unchanged).
So my impression is that upgrade from /e/ nougat (official) to /e/ pie (unofficial) seems to work if phone is not encrypted.
Sidenode: /e/ nougat only seems to install on phone with preinstalled stock rom (FPOS), oreo and pie even on completely factory reset device (without any system preinstalled).