FB> Considering the latest backup is based on Mystic 1.07.3, I was wondering
 FB> about the upgrade procedure: Any recommendations?

It is possible to upgrade from 1.07.3, but that version is around 17 years old
so it would be a lot of work.  You have to upgrade to 1.08, 1.09, 1.10, 1.11,
and then if you'd like to upgrade and run the 1.12 alphas you'll have even more
work to do.

I would guess it would just be easier and/or more beneficial to download a
fresh 1.12 alpha and then reconfigure your system from scratch using things
from your 1.07.3 system that you wish to keep (display files, etc).

--- Mystic BBS v1.12 A23 (Windows)