TimeMachine backup broken after upgrading
The problem with "… Time Machine must create a new backup …" is an old annoying one, see one of many discussions on apple, from 2016.
Apple doesn't seem to be interested.
You did well in finding a good tutorial on jd-powered.net. 

However, as you wrote you accidentally started a new backup, that will have deleted all the old backups.
When I had the recurring problem in 2016 and 2017 I moved the whole old backup over from my time capsule to a spare disk before restarting a new Time machine backup. Moving them over wired Ethernet took days, literally. (Btw: I am not too sure that my old safeguarded backups are still usable.)