Let's say you have a 1.7.4 villager with all trades unlocked(assume a Librarian type with all 9 trades), if you update to the snapshot and trade with him his last trade, he unlocks new ones.

Also, a 1.7.4 villager may randomly turn into a leather worker let's say, just from updating. That also needs to be fixed.

tl;dr Make sure current villagers don't get affected at all in 1.8