Old style plasterboard?

Its easier and cost efficient to take down the plaster and put up drywall. In the future any repairs you need you wont' have to deal with the mess and hastle. It will also give you a chance to look at any problems lying around that you can fix now. Plaster has a different texture and its thicker than drywall, so you will have a time trying to taper out the edges where its not noticable.