Carol, this is the way we did ours. It doesn't matter if it's been painted or not, it all comes off easily after you dampen it. It is a messy job!!
Hi Carol, there is now nice way to do this it is messy this vid should help
Be sure to cover all your furniture or take it out of the room. Lay a tarp or plastic on the floor. You can use a spray bottle and dampen as you go. Use a flat scraper. You may want to wear a mask and cover your hair. utube probably has a video.
Spray it with a water bottle and scrape scrape scrape.
One of my homes had Gunite popcorn and it cannot be scraped, period. It has to be either filled in, or veneered w/ 1/4” drywall.