The difference between the PT ear and the OT era is that a fundamental shift occurred which turned a Republic into an Empire. This also changed the entire design aesthetic that the Republic had versus the Empire which was more about dominating those around them. So yes, much of the design changes are justifiably different.
The X-Wing, on the other hand, is a symbol of freedom. You don't change your symbol of freedom to look vastly different from how it looked when it was doing the things that made it that very symbol.