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

In addition to the advice already given, I would highly recommend using PPG DBC basecoat. PPG formulas are much closer and have more formula variants to choose from than other paint systems. Take the bad fender or gas lid to the paint shop, have them match it and do a spray out.

The belt moulding usually is attached with plastic clips that you will have to replace after removal if they are available separately. If not you may have to purchase the entire mldg if that is the only way to get clips.

You didn't mention the paint code for the car. If it is a 3 stage paint you should plan on blending the hood and rear door or quarter, whichever is the next adjacent. If just 2 stage metallic I would at least blend the rear door or quarter, and would jamb the replacements out, hang them on the car and do the outer panel painting and blending that way.

If it is a solid color with no metallic you may get by with just panel painting the replacement parts if you have a really good match. Totally worth it to spend the money for the PPG DBC for this alone.