I have a '98 S-10 That has factory paint and I guess a clear coat. Can I just scuff it up and paint or just how much paint removing do I have to do? I want to change it from green to black.

All it needs is a scuffing. Many shops use the combination of scotch bright pads, scuffing compound, and water to prep pre-painted surfaces.

With the texture in factory paint jobs, I like to hit the car with 800 wet.
Scotchbrite doesn't level a surface, only scuffs. I like a nice smooth flat surface before painting. use the scotch brite on the areas you can't sand.