We are replacing the tile around our tub. Its a cast iron tub, still in perfect condition so we'll keep the tub and install new tile. We removed the old tile and most of the wall, which was a greenboard drywall. The problem is, it looks like they put the greenboard up before installing the tub, so the greenboard runs all the way down to the subfloor behind the tub. I think this will difficult to get to without removing the tub. I think we could, with some effort get most of the old greenboard out of there but the problem is, then there will be a 1/2" space between the studs and the tub. So if I install the cement board over the studs, it will line up behind, not over the flange of the tub. Not sure what to do about that, I thought the cement board is supposed to be installed 1/4" above the flange, so that any water that penetrated through the grout may just drip down off the wall on to the flange and into the tub. If we install it so that it lies behind the flange, would that cause problems? Apparently, the the old tiles lay over the flange, so we didn't realize any of this until we pulled the tiles off the wall.We don't really want to pull the tub out of there and move the studs, is there any other way to do this properly? (Sorry for the length, just didn't know how else to describe it.)