As one of the "chosen"
, I actually have MC 10, 11.xxx, & 12 installed. They each have their own individual database.
When I upgraded from MC10 to 11.xxx, I simply did a library restore directly from the last MC10 backup zip in the MC10 Library Backup folder. MC 11.x updates the backup for the new version in the process and creates a new default backup in the MC11.x backup folder. The process for MC12 is the same.
Whichever MC is in focus on the desktop is the one that would be updated, (with v11.xxx and v12, MC10 will not open concurrently with another version although it can live on the same computer).
During the beta process, I'm still doing a lot of my tagging in MC11.1, then I do a backup and restore the MC12 db from that file. What you can't do, is go backwards, ie: restore 11.xxx from v12 or restore MC10 from 11.xxx, etc.
Once you upgrade, there is no need to keep the previous version or it's database. The new MC db will contain everything that was in the previous version.