How do we know when a new version will make DB changes?

You'd have to track Kodi development to spot the schema changes that result in DB 'version' bumps. If you're using MariaDB the DB versions are now different for K19 and K20 but that only results in you having two active DB versions. You will have to scrap content twice (once for each version) and might end up with occasional differences in content due to changes in scrapers outputting different results, but overall things coexist (in ignorance of each other) and the only thing that doesn't work is tracking of watched/play status. I run in the same way due to ongoing testing and do an occasional "marking things watched" run on the K20 database (as K19 is still the family daily-driver where the changes accumulate). However my kids mostly watch the same stuff over and over so I don't make too much effort to keep things in sync as they'll quickly catch up once K20 becomes the main DB.