Projeqtor compatibility with PHP installations for upgrades and rollback

Hi, first post here, so I may have missed some existing information but did the search for rollback and noting came back.
On the note of registration, the niggly bit - captcha does not work in firefox browser - no way of registering or communicating. This one is from Safari on Ipad so it worked.
Issue of installation: Projeqtor v8.3.6 stopped working after Hostgator upgraded to PHP 7.4 from 5.6 - I asked them to put php 5.6 back and PQ works for now.
I sent them in chat the list of pre-requisites from the installation instructions but no comment on that. Then I noticed that About PQ shows a number of external libraries and versions used for the PQ.
Not easy to work out a compete environment requied for a given release version.
My installation is a few releases behind and I am happy to upgrade to v9 but have no clear path to ensure that:
1. I have all components set up properly for php 7.4 - what is the correlation between php version and PQ versions?
2. Have no roll-back procedure in case I upgrade to a higher version of PQ and have lo go back to a lower version of PQ to continue.
3. Have concerns about the database structure upgrade with new PQ releases - best would be to state: no DB structure upgrade

The product is great and I would be happy to contribute more improvements for critical professional use at a better level than some expensive commercial platforms that are easily corruptable.

Best regards.

ArthurK