Minecraft - villager no longer changes job?

Moin, I healed a villager zombie and turned him into a librarian, now I've got him on master and I don't need mending anymore. I took down his lecturers, but he still remained a librarian. How can I change his job now?

You can't change his job once you've traded with him. However, you can build yourself a Villager Breeder and you have unlimited Villager

How can I change his job now?

Not at all. Villagers keep their jobs (including trades) forever once you have traded them.

Just make yourself another new librarian.