I have been collecting Richard's art for over a decade now and I am always surprised to find how much he continued to change, manipulate, or downright destroy his art (cut to pieces for mosaics in other work) after it was originally created. Here are some examples that show how he would often alter his work for obvious aesthetic reasons (as opposed to restoration of damaged pieces), sometimes years after the original painting was done.
Tales of Love and Horror
Here Powers did some slight tweaks on the original art, most notably removing the "centipede bra", which perhaps was included originally for censorship reasons.
Sometimes Never/Symphonie Fantastique
Here is an example of how Powers would re-purpose art by altering it to fit a different theme. He eliminated some obvious SF elements from the book cover, shifting the painting into a more purely "fantastique" tone.
Tau Zero
In this example, Powers completely painted over the original painting (seen on the book cover), using it as a guide. At first glance the paintings may appear identical, but flipping back and forth between them reveals they are actually totally different. This "redux" was done 3 years after the original painting (which was done in '76). The "improved" version includes a new signature dated '79. The original signature is still barely visible through the new paint.
Star Science Fiction
Powers evidently found value in much of his commercial art beyond their intended purposes as book covers. Indeed, they frequently where very close to his purely abstract fine art. Here he cropped the art (which he often did to improve composition away from the typical "book cover" format). He then painted over the piece to simplify and "knock back" certain elements and reduce contrast and saturation.