Transition from KERDI membrane backed, to not backed
I like to use Denshield for tiling bathroom surrounds. It is the same thickness as the drywall and it comes with a waterproof membrane already installed on it.  This means that there is no transition disparity between the drywall and the DenShield. 

You can run the final row of tile over the joint between the Denshield  and of the drywall.  You need to use a fiber mesh tape and a liquid waterproof membrane over the seams and  corners any of the screwheads but the rest of the field is already waterproof and textured to except tile.

You would remove the existing sheet rock where you are going to tile and replace it with the denshield.