After tearing down the existing walls in my shower enclosure, I found that in the original installation, the contractor put up the wonder board walls first, then poured the concrete floor, so now I have nearly a 3/4" gap between the stud wall and the finished floor. Should I just leave this gap there an bring the new backer board down to the level of the floor (Option A)? Or should I fill in this gap with concrete before hanging the new backer board (Option B)?
There are parts of the floor where the old backer board could not be removed, so I'm not able to push the new backer board down into the space uniformly. The shower pan membrane is under the concrete floor and extends up the wall about 8", so I feel ok if water gets into the space.
dry set mortar would likely be appropriate for this task...and you definitely dont want a ditch there, that was a terrible install.
i think kerdi set would work well here (just my preference for unmodified)....and kerdi-set is recommended by schluter to extend a kerdi pan if you need too....although not your exact application, it seems appropriate.
dont let that CBU get close to the floor grade (i'm a kerdi fan, so I would just do that for a bullet proof install).