Migration with version upgrade and database change

I'm just looking for some pointers here as a more or less nu-be.We tossed up a gitlab version of mattermost to test it out and decided that we like it. We want to keep going with it. The plan is to run a stand alone community version the end up with a paid for version.

BUT...

I've looking for a migration path where we take the gitlab version (3.4.0 postgres) and move off that server and to a standalone version running 3.7.0(3.7.3) mysql.

Talking about changing from a horse to a zebra mid stream.

Suggestions and to how to proceed?

Regards, Ethan