Making hard copies from plaster molds?

I have some ultracal molds that I need to pull hard copies from so I can clean up and remold to cast in a different material than what I have been using. I have heard I can put in fiberglass and break the molds open which would leave me with a FG copy of the piece. Since, obviously, this is a very risky procedure I need to know the best way to go about doing this and the materials to use. Any help you guys can give me who have been doing this a while it would be a great help to me. Thank you for all your input.

-Matt

First, be sure that all the water from the plaster has evaporated. I spray it with two coats of automotive spray paint and put 3 coats of wax over that (I like High temp TR mold release wax).

If you've done it right, you may even be able to save the mold depending on the undercuts.