I am in the process of re-tiling the walls in a shower. (I am not replacing the tub as it is in good condition) In the demo process I discovered that it looks like behind the tile is green board over dry wall. There does not appear to be any water damage. I am assuming that it is recommended to replace the green board with hardi backer or something similar. I was wondering if I should take it down to the studs or if it would be ok to re-apply the hardi backer over the drywall.