I'm not sure I buy either Peter Connolly's or Adrian Goldsworthy's theories.

Having gaps in a battle line is a potentially lethal flaw, while rank changes which open such gaps is equally dangerous.

Multiple battle lines were used by a great many armies through out history, to replace or relieve exhausted or broken front lines. I imagine the Romans were no different and replaced Hastati with Principes as and when a lull in the fighting gave them a chance to do so, or to quickly fill gaps created by broken Hastati maniples.