Villagers not changing professions.

Dear Mojang, i am trying to get a mending villager in my minecraft world. however, whenever i try to change the trades of a villager, it simply does not work. by this i mean, when i change the profession of a novice to librarian, and remove thier workstation in order to alter thier trades, the villager does not go back to being a novice. nor does it alter its trades. i even have some novices that does seem to change profession at all. please help.