I did some cornice work in summer, and now with winter upon us, I have small cracks between the cornice and the ceiling. So being the bearer of bad news, I'd say restumping will cause some seperation. It's not all doom and gloom though; if you run a new bead of plaster along the cracks, and sand them back, add a coat of paint, they should be fine again. Just more effort though :( I have a room with twisted cornice from when they were put up in the 70s, and is going to give me grief tying into the build-in wardrobe I added. I could recornice the whole room, but then I'm in for some pain removing all the cornice cement. I could replace the ceiling as well, but then I have to stuff around with the insulation again.