What you need to do is scrape the texture off. Wet the ceiling with water from a sprayer and take a putty knife and scrape the surface. This is a huge mess so cover everything. Once you have cleaned the surface, let dry, and sand lighty. You will have all kinds of imperfections. Start applying a skim coat of mud to these areas. Let dry and sand. These will be hard to see, so once you've sanded, apply a primer and look it over. If needed,repeat the skim coat process again. Don't forget to re-primer those areas.