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
Villagers will get a specialzed profession. Old trades are retained, but new new trades are unlocked.
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