I just got done covering up an old popcorn ceiling with new drywall that I finished smooth. I glued and screwed it (just be sure to get longer screws to go through the old layer and get a good bite in the joist). I would definitely agree that you should use 12' sheets if possible to limit your seams and cut the panels a little short (if your walls are already up) so you have room to maneuver the new panels up with a lift (new crown should cover up the gaps). I would also recommend putting additional blocking on your joists so you have a large margin of error when in comes to lining up the edges of your panels. It also helped me to go into the attic and drill holes along the edges of the joists through the old drywall to give me a point of reference when hanging the new panels. I HATE popcorn ceilings, so it was worth it.