Quick note, maybe someone else will find useful.
I had previously upgraded the RPi3 to LE11/Kodi 20. But because of the well known problems with HD content is rolled back to LE9. This left a MyVideos121 on the DB server. Consequently, LE12 upgraded that DB instead of the MyVideos116 DB that I was actually still using.
Easy fix: Stop the new Kodi, drop MyVideos131 and 121 with phpMyAdmin and start Kodi again.
Thanks chewitt for the concise and complete answer. It seems it'll be completely painless. 
Excuse me if this has been answered, but I can't find an definitive answer on me two questions.
I intend to move from a RPi3 on 9.0.2 to RPi4 with 12.0. I use a MariaDB SQL database on a NAS and my files are also stored there. I understand this requires a fresh install. So my questions are.
1. Will the new RPi4 automatically upgrade the Shema when setup to use MariaDB? And,
2. Is there a way to backup the 9.0.2. local setup, plugins, API keys, logins etc. and then restore those on 12.0?
Thanks in advance.
Douw.
