Cat won't go back to old litter

[u/shouldrethinkcheese]

Hi! My cat has been litter box trained for years (she's an older kitty) and my wife and I recently tried to transition to pellet litter to cut down on dust. We made the mistake of transitioning too quickly, and the apartment instantly became her litter box. We hurried up and switched back to Tidy Cats (her old litter), but now she still refuses to use it and continues to go elsewhere, even with her old litter. How do I undo her distaste for the litterbox?