There are one or two little details you'll want to take care of before putting flat paint over semi-gloss. If you want your flat paint to stick to your semi-gloss you'll have to rough it up a bit. In other words, you'll first have to knock the sheen off the semi-gloss. This can take a little extra time and effort on your part, but it is essential for a professional-looking paint job. And keep in mind that flat paint, even supposedly washable flat paint, will never clean up as easily or as well, especially in kitchens and bathrooms. In those two rooms you are almost always better off in the long run painting with a semi-gloss paint.
Mix a cup of TSP cleaner into a gallon of warm water. Wear rubber gloves if you have sensitive skin. With your sponge, wipe down your walls with the TSP and then wipe them again with a sponge dipped in clean water.
Rough up the gloss on your semi-gloss paint. There are two ways to do this. Sand the entire wall with 100-grit sandpaper. Use a pole sander. You do not need to sand to bare wall. All you need to do is to rough up the shine of the semi-gloss paint. The other way is to use a paint brush and apply a deglosser (available at most good hardware stores). You'll spread on a coat of deglosser with your brush and then wipe it off with a clean cloth. Then rub your wall down with a wet sponge frequently dipped in clean water.
Wipe walls with a tack cloth after sanding.
Paint on a coat of good primer, and allow to dry. Paint with a medium-grade (or better) flat paint.
Or, skip the primer and paint two coats of medium-grade (or better) flat paint if that is easier or cheaper.