Walls and Ceilings - Question about cutting plaster over drywall to seem with greenboard from alcove

In my bathroom where I am installing a new Tub/shower, the alcove is getting new wall straight to the ceiling everywhere except on the far tub wall. That side will have to meet up with the existing material which is Plaster over drywall. I need to cut the plaster over drywall nice and straigt from floor to ceiling and meet up with the greenboard coming from the tub alcove. What is the best way to cut the plaster over drywall? I own both a dremel and this thing

http://www.sears.com/sr/javasr/produ...seBVCookie=Yes

Why are you saving one part of a wall to cause yourself this headache? If you tear out the entire alcove, and replace all the wallboard, everything will be smooth. Just wondering.

Because tearing out the entire back wall would recquire me to retile the rest of the bathroom. The reason is this: If you were standing in the alcove with your back to the shower wall side, the wall you would be facing is 30" in the alcove, but extends all the way to wall to your far right and is tiled with a matching pattern of that wall and the three walls it leads to around the toilet and sink and back around to the side of the shower side wall. I am tearing out the whole alcove, but that back wall runs the entire length of the bathroom.

bump - Anyone have any advice at all?