Villagers will change their profession even after having their trades and profession "locked."  the profession chosen by the villager (randomly chosen from nearby workstations) is then locked, along with the trades. 

example: a librarian villager with a mending book trade (in a personal world) randomly changed into a farmer, and regardless if the workstation is placed or not.  the profession was not able to be changed back, but the exp from the librarian was kept.