If you are going to go over the popcorn or textured ceiling with at least a minimum of 1/2 inchsheetrock, you still need to remove the material to get the sheetrock to get up tight to the existing ceiling.
If you do not , then don't expect good results. And it may even fall on your head over time.
I would just remove the texture, and not by sanding, it may contain asbestos.
This has been covered so many times in the past, here is another link. Post "popcorn" in the search box up top for more.