I have painted my son's room with semi-gloss and I am not happy. Can I paint over it with Satin of the same color or do I need to do something to the wall first.

Hello SG,
Obviously, it's less work to just "live with it"! S/Gloss is very cleanable, assuming two coats were applied.

However, if ya just gotta get rid of the shine...wipe on a "liquid-deglosser", aka "liquid-sandpaper". This TEMPORARILY gives a glossier surface some bite.

Check directions, but normally don't wipe an area bigger than you can paint in 15 minutes. If using the same color, 1 coat of Satin should be fine. If a new color, TWO COATS!

Faron

The fumes from a deglosser will be very strong,way stronger than paint thinner. What you need to get is a couple of sanding sponges and lightly sand the walls and then wipe with a damp rag to remove sanding dust and apply the satin latex. This will work just fine and you will not kill a bunch of brain cells along the way.