Removing tile and backer board question

Found that water was getting behind the tile wall on our bathtub. I started to remove the tile and it looks like they used drywall behind the tile and then backer board behind that. I am going to remove all tile and drywall. But if the backer board is intact, can I leave it on and add another layer of waterproof backer board such as Everbuilt water shield. Instead of tile, I am looking at flex stone tub surround. Am I completely out of my mind