How should I prep for a color change over existing good paint?

Talk to the guy at the paint store about the sealer. Sealer is usually epoxy primer that is thinned out more so it goes on smoother. If that's the case with their sealer, that may save you a step. If you don't have any degreaser, get you some of that and some tack rags. They will have some blender that is in a spray bomb, to spray on the point the new clear meets the old clear, so ask about that too. You just hit the edge of the new clear with it and it melts it and the old together. What size is the tip on your spray gun? 1.3 or 1.4 is best for everything you will be spraying.