[mapt]

This is not so much a gradual change, as a recurring theme in coinage through history, with the stability and grasp of a regime/kingdom/empire being related to how much seigniorage (profit) it can currently extract from the economy by putting the leader's face on a costly bit of metal. This may be from one of many periods where the old coinage of an obsolete regime reverted to holding negligible value over its metal value.