Never had a problem painting over gloss or eggshell finishes.

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But prep is necessary. Make sure they are clean, first of all. If the walls are smooth than just run a sheet of 160 grit sandpaper over them to break up the shine. You don't have to sand the finish off, just rough it up a little. If you have textured walls, sand, swirls, whatever, then sanding won't work. There is a product available in professional paint stores called, I believe, Emulsibond. It's a milky liquid you add to the paint to improve the bonding to the old wall. It works very well and is well worth the extra cost.