Here's a long thread with plenty of answers to your question, plus a link to a good article:
I would recommend hanging new drywall. It's easier than it probably seems.
If it has been painted, it will take more time to soak in the water. A steamer might work. We used hot water by connecting the hose to the hot water heater and using the mist setting on the hose sprayer. Don't forget protective masks as it has a small amount of asbestos in it. Most communities consider it hazardous waste, so check to see how/where to dispose of it. Good luck!
Hire a professional...
We just dry-scraped ours. It leaves a tiny bit of texture, but it actually looks pretty good. Just make sure you wear a good mask and goggles.
Mine have been painted and are hard like stucco. I don't think they are going to be able to be scraped off.
purchase a 12" sheetrock knife and mist with water using a chemical sprayer....if it has been painted it will still come down, it just take a little more elbow grease. The sheetrock knife has a super sharp edge that helps break loose the popcorn texture. Have done this many, many times due to being in remodeling. Good luck and remember to wear goggles, cause what is up will come down. Spread plastic on the floor for easy clean-up.
Lowes sells a special tool for this, look in the section where grout and grout utensils are.
@Candicekane100 IMO it isn't worth the money...
@Debra B. if your ceiling was painted with oil, it will be very hard to remove.
I read somewhere you can fill a sprayer bottle with 50/50 mixture of Downy fabric softener and water. Spray on, let sit for a few, and lightly cut and peel off. Should not leave any glue. If it does, spray again, repeat process. No chemicals. No special tools. Good luck.
Mine had been painted - walls & ceilings . I painted on CITRI-STRiP with a paint brush (hardware or walmart stores), let it sit until bubbly (3-24 hours) and it scrapes off in sheets! Totally amazing. Wash with a green scrubby pad and mineral spirits. After doing 3/4 of our home the hard way & dealing with gouges &dust. What a great product!