Lay 6 mil. plastic on the floors and anything it's going to fall on. Mist the whole ceiling down with hot water and a dash of liquid fabric softner, let it sit a while. Scrape it off with a sheetrock knife held at a sharp angle. If it starts to get hard to scrape mist it down again. Once it's all off let it dry for a day then sand the whole ceiling with a sanding pole and med. grit sanding cloth not sand paper. It's sold in the drywall area at any Lowes HD or hardware store. It looks like screening with grit on it. Once it's all sanded wipe it down with a broom then fill in just the low spots with drywall compound and tape all the seams if it was not done before the texturing. It has to to be perfect with no low or high spots. No paint is going to hide the flaws. Prime it twice then paint with ceiling paint.
It's not hard to do just messy.
Do not try and just level this stuff off, way to much work to get it perfect.
Do not try and just go over it with sheetrock, it would never lay flat.