Upgrade of my Debian 8 server was long overdue; however, went headlong into it and ran into problems the first time round (i.e. MariaDB wouldn’t start properly. So I restored the server back to 8 (thanks Vultr for snapshots!) and tried again. This is what worked the second time round:

1. Shutdown MySQL/MariaDB before attempting to upgrade Debian (a step I noticed after the failed first attempt; Section “Upgrading to MariaDB 10.1 from MySQL 5.5” of MariaDB’s documentation “Moving from MySQL to MariaDB in Debian 9”).
2. Follow Steps 2 to 5 of phoenixNap’s guide– it had the easiest sources.list edits for switching to Debian 9 repositories.

As the primary function of this server is to serve PHP applications, it was also an opportune to upgrade the PHP version as WordPress will soon require at least version 7.2.

1. Follow Steps 1 to 3 of Computing for Geeks guide – this will upgrade PHP to version 7.4
2. Follow Section “Activating New PHP Version in Apache” in JiyuuLife’s guide for the quickest way for Apache to use the new 7.4 instead of the previous version (7.0 for Debian 8).