Look at the automobiles from the fifties, with all their curves, and then by the seventies they’re all boxy.
But it still doesn’t make sense.
1950s cars are all curvy, 70s they get more boxy.
But why is it that all the “1950s” ships in the PT look like they just got driven off the lot, while the “1970s” ships in the OT look like they’ve been around for more than 20 years? They’re all dirty, and worn-in.
The general idea is that since the rebels didn’t have a “military budget” they simply got whatever they could get their hands on, and aesthetics wasn’t exactly a priority either. The Y-Wings f.ex. are canonically old de-commissioned CW-era fighters that had been stripped down to their bare essential even before they stole them. Also the OT is after all 20-ish years post PT, so even if some of the more “modern” boxy designs are only 10 or so years old, that’s plenty of time for them to end up looking old and worn in by a rebellion that literally hides out in caves and has minimal equipment for maintenance. The degree of wear also varies from ship to ship. The Tantive IV and the Calamari cruisers are much more well maintained as they were in the service of their respective militaries before joining the rebellion, but on the other hand something like the Nebulon-B frigate is meant to be stolen imperial ships much much of their outer plating stripped off (similar to the Y-Wings.)
As far as retcons go I think they’ve mostly done a good job of it.