Accidentally Upgrading to Master (4.1dev ALPHA)

A recent comment on my channel reported an error message that can result from upgrading to the "master" branch of Moodle. Where it's not the end of the world, this is irreversible and is rather easy to do.
Here is the message:
ERROR!!! The code you are using is OLDER than the version that made these databases!
If you ever find that you have accidentally upgraded to the development version, here's why it happened and what you can do to retrieve your courses and data.

[@kakhaberrevazishvili7736]

Hi WiseCat! I have error in moodle upgrade. can i solve it, or need to clear install moodle?
error: Your local changes to the following files would be overwritten by merge:

[@Wise-Cat]

This means that you have some changes to your Moodle code. Do you want to keep those changes or are you free to discard the changes? Well, that will depend on what changes have been made. Unfortunately this is very much a case-by-case type issue and I don't have a simple answer that will be applicable to all.
Well, except this... Make backups before moving forward. Multiple backups!
Good luck!