Do you use flat or semi gloss paint for the ceilings?

You will have no problem painting satin over top of flat paint. Just remember to use the appropriate paint that will stick to the old coating. If the old coating is latex (water based) you can go over it with latex or alkyd (oil based) paint. If the old coating is oil based you can only go over it with oil based paint or a hi performance latex that is specialy made to go over oil. Most latex will not stick to oil based paint.