Inspired by watching a video recently where the person making it thought it was inappropriate to refer to the roman infantry at Zama as "legionaries".
Wiki also seems to think that it only refers to post-marian soldiers.
My limited googling says that "legionarius" simply means "of a legion".
Are you asking about the usage at the time, about later Roman historians or about usage amongst historians today?
Livy & friends have no qualms whatsoever to speak of legions and legionnaires even during the early republic, nvm Marius.
Also keep in mind that the legions got their name from their being a levy army early in their history.