It seems as though all of the movies released before the advent of the DVD have been given new cover “art” for their DVD release. For example, the original artwork for the movie “How to Beat the High Cost of Living” had three naked ladies standing together in a barrel which visually symbolized their financial difficulties while at the same time enticing with sex. For the DVD release, the cover has simply a photo of the three actresses together fully clothed.
Looking at DVD releases of other older movies, the poster art is generally replaced by photos of the actors. I wish they didn’t do this because I think the original poster art is most always better than the photos.
Sometimes they do it because they need to rearrange the aspect ratio of the poster design to fit the case dimensions, and allow for additional information. And it also has to appeal to modern audiences - there’s no doubt modern graphic design is often an improvement on whatever the hell they were smoking in the past.
I think in the end it just comes down to some distributor saying “we can do a better job than that old thing” and then proving themselves to be completely wrong.