Villagers can change profession regardless of what time it is

1 - Start a new world

2 - Set the time to 11500 and wait until night

3 - Change the villager's profession

If you make the villager sleep and break the bed they're sleeping in they will no longer be able to change profession until daytime

I did this in a superflat that, when i created it, had doDaylightCycle set to False, but i later turned it back on. Not sure if this affects things