You might be interested in some previous threads on Roman weaponry and equipment changes:

• Why did the Roman shield design change from a squared shield to an oval? - 22 comments, over 1 years old.

• The topmost comment directly addresses the issue, though there is some debate on the matter later in the comment thread below it with primary source quotations.

• Why did the Scutum fall out of use? - 5 comments, over 4 months old.

• This thread addresses whether there actually was a standard shield design at any point during the reign of the Roman Republic/Empire and also discusses the capability to even draw such conclusions with the evidence available.