I though this was going to be a nightmare, but getting the content moved from 2.5 to 3.3 was the easiest part, using the BlogML export and import. The tricky part was adapting the Standard theme to match my previously customized one. A straight copy of the theme folder did not work.
This will do for now.
For years I've used a database build script framework written in-house to recompile all schema objects and run unit tests. However, the actual DDL code to migrate a schema from one version to the next was always left as a hand-rolled script that is executed up-front before the normal build process.
A colleague introduced me to Flyway DB and I am now a fan. The introduction, documentation, and tutorial material is top-notch.
Check it out here: https://flywaydb.org/
Although it has an API for integration, I specifically like the command-line utility version: https://flywaydb.org/getstarted/firststeps/commandline