IIRC, Batman TAS was edited on video, so they’d have to locate all the film elements and rescan them. It might get more tricky if any digital effects were used to fix animation mistakes not caught until the footage came back from overseas.

There were a couple episodes where the you could see an amazing amount of dust dancing on the cels as they were photographed. That’s going to be even more noticeable in HD. I can’t imagine there’s any way to clean that up without doing more harm than good.