I did a few tests today to see what would happen to the villagers born in 1.7 when switching to 1.8
All four farmers, smiths, librarians and butchers picked a profession and new 1.8 trades in addition to their old trades (no bug here).
But all my already unlocked priests were turned into clerics and lose their old trades. I don't really know why.
This is clearly a bug since it only happens to these particular villagers.
Generate a world in 1.7, spawn some villagers, then reload this world in 14w06b and note that they all but priests kept their old trades
Edit : it doesn't seem to happen anymore.