This is never easy because the ceilings are always different.  Sometimes the best way to handle a situation such as this is to mask off the walls and floors complete if necessary,  then wet the popcorn and let it sit for about 5-7 minutes.  to a test spot to see how it comes off.  If needed, wet it one more time and usually it can come off almost as easy as non painted popcorn.  You only have to be careful and not tear any paper on the drywall.