Painting over a faux finish is not difficult. The key, like most painting, is in the prep work. In addition to the standard prep that any good paint job would require, what follows are the additional steps required to ensure success.
The biggest challenge when painting over a faux finish is often dealing with any texture that the faux created when it was applied. Occasionally people like the texture and just want to paint over it but if you want to get rid of the texture then the first step is to scrape the wall with a broad putty knife. Make sure to put down a drop cloth first. After scraping the walls with the putty knife you will want to sand the walls. You can use a pole sander or go with an electric orbital sander. I prefer to use an orbital sander and one of the nice things is you can set it up with a shop vac attachment to vacuum up all the dust. If you go the orbital sander route use a fine filter paper bag in your shop vac. The type for drywall dust and a HEPA filter. If you use the bag it will help keep your HEPA filter cleaner and the HEPA filter really helps keep the air clean.
After sanding, I would use about 100 grit paper by the way, you may need to skim coat the walls with drywall mud if the walls are not smooth enough or if there are other imperfections that need to be addressed. If you need to skim a large area I would recommend rolling a slightly thinned down drywall mud on the walls using a painting frame and 3/4 inch nap cover. Using a 5 gallon bucket and paint grid works best. Work in sections and after rolling out the material draw it down using a broad drywall knife. The skimmed areas will need to be sanded and primed when dry. You may need to mud additional areas after sanding the skim coated walls to get them just right.
Always prime the walls. This will seal the surface, deal with potential compatibility issues between the top coat and the faux and it will ensure good adhesion of the top coat. A great primer is Seal Grip.
Painting over a faux finish is not difficult. It just takes a few more steps but the end results will be great.