That can't be right. The Normans as a people played a major part in European history from at least the year 911 through the Hundred Years War (1337-1453). I've never seen the faintest indication that they weren't still called Normans after leaving Normandy, by themselves and others, e.g when invading England and Italy.

I guess you meant more that the term fell out of use in England at some point, and Norman-descended people in England called themselves English. But even so, Norman is still a recognisable ethnicity - norse people who settled in France, spread by conquest into England, Malta etc. It could be a useful contemporary or historical label even if those people at a given time called themselves something else.