Everything in moodle code ... core or addons ... has a version reference now in respective directories. Core changes requires plugins to be updated as well ... as evidenced by your first posting about the problem (depreciated).
It is somewhat amazing that your site hasn't had issues when it was a 3.5.
Always, before upgrading, make full site backup ... that's code, DB, and minimally moodledata/filedir.
Then check the Environment and update the component. That will show if you have required things to check.
Addons are not part of the environment check. For checking those the Admin user (ie, you) must check plugins at Moodle.org to see if there is a compatible version for the core version you are upgrading to.
This thread, BTW, is in the developer forum ... meant for folks who program Moodle code but they may NOT work for Moodle.org. Folks in here generally don't respond to issues concerning upgrade failures.
Jon is right ... posting in multiple forums does make 'following the conversation' more difficult. Granted you were in a panic something of which might have been avoided if following a regular process/procedure for upgrading.
And, you do know that persons who respond here in these forums are 'volunteers' and do not have any obligation (contractual or otherwise) to those who post questions on how to fix, etc.
Having said all that ... glad you have got it fixed.
'spirit of sharing', Ken