I would go and ask the home improvement store if they have a special primer for that that will help dull the surface. KILZ is a gret primer. I know someone who did this without priming and the new paint actually bubbled and fell off.
Here is some good advice I came across:
When repainting glossy surfaces, sufficient cleaning materials must be used to dull surfaces, or they should be lightly sanded. An alternative to sanding is the use of a liquid cleaning/dulling solvent. High-gloss surfaces typically do not provide good adhesion for new coats of paint.
Painting over wallpaper is not recommended; the old covering should be removed. Once painted, wallpaper is extremely difficult to remove.
Do's of Interior Painting
* Wash all grease and dirt off walls and woodwork.
* Patch cracks in walls and ceilings before painting.
* Seal all new surfaces with a primer.
* Scrape off all loose paint and sand the surface to a smooth finish.
* Stir paint thoroughly before any applications.
* Allow new plaster to dry before painting.
* Properly ventilate area to be painted.
Don'ts of Interior Painting
* Don't expect good results on dirty surfaces.
* Don't paint over a damp surface with oil-base paints.
* Don't apply the second coat of paint until the first coat has dried properly.
* Don't sand woodwork across the grain.
* Don't change cans of paint in the middle of a wall area.
* Don't add thinner to the product unless directions call for it.