Not by any means being an authority on the subject - in fact a complete novice, but trying to bit by bit add to my knowledge of medieval genealogy - the slave or serf status didn't last, disappearing not long after the Norman Conquest.
Amongst the poorer people there were free as well as unfree tenants, but the bargaining position of the unfree tenants altered hugely after the ravages of the Black Death (1348-50) - supply and demand, with the population massively depleted.
The point that is being made about whether "once an ag. lab., always an ag. lab." is that fortunes could fluctuate for any family either upwards or downwards, and there's nothing to stop the possibility of a family of ag. labs. once having been quite well to do and perhaps getting a mention in the medieval records of the time...
I've also spotted the fact that if your genealogy is in the Durham area, you're luckier than most at the moment, for a database of 11,000 names is being put together from the 9th to the 16thC, commemorated in the Liber Vitae of Durham. Not sure whether there's an internet link for this yet, though.
keith