Well you can scrape it all off, assuming it was never painted since the original application, but that will be a vigorous job… however, if indeed it has once been painted then you can't remove it…

If it was previously painted, you could have the ceiling replastered to give a smooth finish, then prime and paint it…

We have painted over the (never before painted) pop-corn on many jobs with regular paint with successful results, that is, if you want to still have the pop-corn finish, but with a different color, or you can paint over it using a texture paint, which will hide the flocking but give a texture finish…

Rolling a previously unpainted pop-corn ceiling is a bit tricky since the flocking will tend to come off with the roller… but it can be done using slightly thinned paint and a 3/4 nap cover… and rolling very slowly applying very little pressure and frequently reloading the roller with more paint…

For best results, mask off necessary surfaces, then spray the ceiling using an airless system…