Visvanath Ratnaweera

My boiler plate answer to for somebody coming from releases 3.9-3.11.

- Release 4.1 is LTS, security supported longer than 4.2 and 4.3.

- Do a trial upgrade first.

Your plug-ins are safe (from memory, visit their https://moodle.org/plugins/ pages and see for yourself).

BTW here is the documentation Upgrading (you can "hack" the URL to see /401/, /402/ etc.)

Oh yes, the system software. Ref; http://www.syndrega.ch/blog/#php-and-dbms-compatibility-of-major-moodle-releases.

You have an advanced architecture which is not common in this forum. Those who tried to answer are all regulars with experience in everything Moodle including large installations. If they can't answer your questions, you have dwindling chances here. Try external resources, like Moodle Partners as Marcus proposed or try to contactA new "Large scale Moodle deployment" support group . Before you ask, No, I'm not in that. So you have to ask them.

Never used Turnkey. Its web site says Debian compatible. Therefore Installing Moodle on Debian based distributions should have the information.

You need to study:
- Course backup
- Course restore
- Site backup
- Site restore
- Moodle migration
- Upgrading