If I may wade in on this...Whatever you replace your plaster with is inferior. There is no other fact you need to know. If your plaster (which has a track record of over 12,000 years--maybe not yours)is still up it can be fixed cost effectively. Plaster installed before the 1920's is generally a lime sand hair mixture. This material can get wet and dry out and still be in great shape. So unless you wish the value of your real estate to go down, keep your plaster.