i'm replacing the tiles in my tub area and i'm changing the existing wall to cement board. how do i join it with the rest of the bathroom wall that's texture?
I wouldn't. I would just cut away the area where the backerboard is going and remove that piece of the drywall and make it flush with the rest of the wall.
find the studs to screw into and use liquid nails
If you are putting in cement board to put up tile, you need to remove the drywall from the area that you will be hanging the board in. The backerboard should go directly on the stud. This is for two reasons: 1) The backerboard, as you know, is heavy. If you have drywall inbetween, the screws will not hold as well. 2) If you put it over the drywall, it will stick out from the wall, and the edge of your tile will not be flush with the existing wall. In my opinion, demo is the funnest part of home improvement! Just take a hammer to the existing drywall, knock a hole in it and pull hard. You may need to use a SawzAll (reciprocating saw) to get a nice edge, but make sure you know whats behind the wall before you start cutting. Stop demoing at a stud on either side, so that you have something to screw the cement board to. As a side note: I have tiled over drywall with no ill effects. Just scrape it smooth, apply the mastic, and go. Cement board is safer in a bathroom, but really only a requirement for floor tile. Good luck.
Use backerboard screws, (or galvanized nails/screws) If you want overkill use stainless screws.. Some say you should 1st install a waterproof membrane to protect the wood framing from moisture ie Tyvek or felt paper etc. backerboard will not be damaged by water but it is not a waterprrof barrier. I don't know that for sure but better safe than sorry. Seams in wonderboard/hardiboard etc should be taped w/ fibreglass tape and tile adhesive. Edit: I just read your additioanl comments. I would extend the tile beyond the backerboard seams.
