But the 1.1.42 BP lib had then been upgraded to 1.1.45 and could not be used any longer.

I heard on discord that downgradning BPs is not supported.
If downgrading BPs is not supported, then I really think that you should make a backupfile every time the game version change. So going from 1.1.42 to 45. The game should create a "backup-1.1.42" of the blueprints that can be reuse when downgrading the game.
(Note: To solve my problem, I was forced to start using 1.1.45 on my 370 hour modded "Pure Angels" game. I am still not sure if I will have a problem now with this change. I would have preferred to stay on 1.1.42 with all my blueprints.)