Converted units are frozen in the tech level they had when they were converted. There's no way to upgrade or improve them, but they will get some abilities (e.g. an enemy villager that has been converted in the dark age will be able to build a stable once you've advanced to the feudal age; even if the enemy culture doesn't even have stables).

It appears that build/research/train abilities are the only thing that is not affected by the "conversion tech lock".
