What is the proper protocol when painting a room that has already been painted before?

Ex: a room has semi gloss, or eggshell paint (any color), and the client wants the room to be repainted the same color just to refresh the look.

I've been understanding that you should sand the walls down to allow the paint to grip the surface, tack cloth or dry dust them, and then go straight to paint (using Sherwin Williams ProMar 200). Is this the way? Also, is primer necessary or no?