On medieval rents ruling out any savings, you're probably thinking of the rack-renting of the Border clearances in the 1700s. There was room for upward mobility

On medieval rents ruling out any savings, you're probably thinking of the rack-renting of the Border clearances in the 1700s. There was room for upward mobility in the Middle Ages; for one example, look up the Paston letters and the biographies of the family, who rose from yeomen to gentry and I think minor nobility. Yeomen were freeholders, approximately, but we have records of serfs rising to yeoman status. See Frances and Joseph Gies, Marriage and Family in the Middle Ages, for a great deal of discussion in passing; or look up A Farewell to Alms, although the situation wasn't as rosy as that book claims.