Hmm, sorry to disagree with you folks
but in the FAQ there is this entry . .
Can a follower be moved from Carcassonne City to a farm with a barn - and if so, how many points does a player earn (1 or 3)?
Yes, that is allowed: the farmer will then be scored immediately, and so scores only 1 point per city and is (importantly) not on the farm during the final scoring proper.
Seems to me that this implies that a meeple can move to the barn farm at game end (i.e. "when it is scored"); score one per city (unless another player steals majority by moving a big meeple from CC); and then be removed for final scoring "proper" of the farm.