If you're looking for a quick"Maaco" type of job, you can leave everything on and mask around it(not advisable), or strip it down and do it right the first time. Everything should be removed, especially if you're doing a color change.
I'm sure others will chime in, but keep this in mind, any good painter will tell you, 95% of the end result is in the prep. This is not the area to cut corners, and you get what you pay for. Good Luck.