Replacing trim set in horse hair plaster
Yeah it is more brittle than sheetrock but still strong stuff.  Start with scoring the paint/caulk around the casing with a utility knife, then use a pry bar as normal, except use a board or plywood between your prybar and the wall.  That way you won't have the pry bar indent into the plaster as you pull the wood off.  Work your way around the door casing, prying it a little at a time.  Once you see where the nails are you can pry directly under that point, or hit the face of it with a hammer and sometimes that makes the nails pop out enough you can pry them out through the face.