As the series progressed, a lot of the vehicles (and clothing) became standardized to make continuity easier. That still didn't stop AMC Matadors occasionally appearing in chase sequences in later seasons.
I'm guessing that rather than swapping the interiors, they probably swapped the front grille/lights from '77/'78 cars onto '75/'76 cars so that they looked the same from the outside. Of course, the makers probably had no idea that we'd still be watching the show 40 years later and spotting these little inconsistencies!