If you have a popcorn ceiling then you'll need to scrape it down,messy.
A knockdown ceiling is different. Orange peel is a texture of it's own. Knockdown is a process that follows the application of the new finish and then after the ceiling is allowed to dry (some-what) a flex blade or trowel (I use a flexable splatter shield, it's wide and can be used with ease.) the top edge (High points) of the ceiling are then "Knocked Down" giving a nice grained finish.
Labor intensive, but very nice when done properly.
Consider using a "Bull-nose" corner bead if you have exposed edges. This softens the over-all look of the job.