Wash with TCP using 2-3 tablespoons in 2 gallons. It will
slightly degloss the semi-gloss paint and provide a tooth
for the new paint to stick. You need to do this no matter
what kind of paint you put on top, and you don't need
primer. I just use an old wash cloth. I use only
semi-gloss on the walls in my house and have had no problems
over a 27 year period of ownership. If the color change
is great or you go from dark color to light, you probably
will need two coats.