Take a close look at your ceiling, especially corners and edges. If the ceiling is not adhered well to the drywall, the additional texture will simply add more weight, and both finished may pull right off. Bathrooms and rooms with moisture often have peeling texture problems. Spraying over these ceilings is not suggested, and an alternative option to change your ceiling is recommended.
If you are adding additional popcorn ceiling texture, the two may blend well. The look will be a heavier version of the texture you currently have present. However, of the texture is a knock down plaster, it may not be pleasant to the eye. You will have difficulty smoothing the new texture smooth, because of the peaks of the current popcorn texture.
Consider removing your popcorn ceiling, and then retexturing the surface. If the ceiling has not been painted, moisten with a blend of water and dish soap. Use a water spray pump to saturate the ceiling. Allow the water to soak in for about 15 minutes. Then begin to scrape the ceiling with a putty knife. If your popcorn ceiling is painted, it will not absorb any water. Simply begin scraping away at the texture. Be warned, this process is extremely messy due to clumps of falling plaster.
If your home was built in the 1970s or before, the popcorn ceiling should first be tested for asbestos. You can hire a local laboratory to test your popcorn ceiling. If Asbestos are present,and you begin scrapping the popcorn material you may inhale much of the dust. You are then at risk for mesothelioma cancer or other respiratory diseases if you breath in asbestos material.