Do you have to do anything before painting satin over flat paint?

I have done this in the past with little prep; just make sure that everything is clean and smooth but flat paint is like a primer.

No prep other that clean the surface area (wall / room / object) so that the paint is being added to the surface and not the dirty wall this will make your painted wall last longer and not need to be repainted as often.

So long as it is clean, you should be good to go.

Agree with all of the suggestions so far.

The thing you want to do prior to painting a satin surface to a flat painted surface is same as it would be if reversed. Do repair any holes or cracks in the surface, Do sand any areas you have repaired and any areas of the surface that are uneven. Be sure to make sure that all are completely free dust or hairs. If necessary, use a very slightly damp cloth to softly skim the surfaces you want to paint... This way, you will be positive there is no debris left on surface, prior to applying the paint. The next best thing to do is to make sure you have enough light in the room, when you are painting, so that you can paint all of your walls/furniture/floor evenly by either paint brush or a paint roller. Good Luck!