This times, well, a lot 😁.
So many people assume they can only paint over the bare wood and get into a world of needless work trying to achieve it.
The best surface to paint over is a good condition paint surface.
If you take it back to bare wood then you need to grain/pore fill and then get a layer of sanding sealer or primer on there to enable you to flat (sand) it back to a smooth surface ready for paint. Quite often you will need to repeat the primer process before you end up with a nice smooth, level surface. All that work to get back to where you started, minus the shine. Also along the way, unless very disciplined, you can introduce unevenness into the surface of the body. Once painted this will show up as a wobbly finish in the shine, leading to even more work to flat and polish.
If your finish now is good then just take the shine off with some scotchbrite or 800-1000 grit wet'n'dry used wet. It really doesn't need scratching up a lot or again, you'll just be making work for yourself to overcome those scratches in your finish.
If you have any dents or scratches they can be filled with a suitable filler and then those areas will need a coat of primer to stop them showing through your finished paint. Car stopper, which is a very fine filler, rather than wood filler is best as you're not repairing wood now, just a paint surface. You can prime the whole body if you want but you don't need to, just the repairs.
Let the primer dry and flat that down with the wet'n'dry. You can paint straight over the primer after it's flashed off (what they call wet on wet painting) but there's a chance the repair will be visible in the paint finish.
The only reason to completely remove the finish is if you plan on relicing and don't want the original colour showing through, if you're worried about chipping it and seeing the original colour, or if the finish is just in too bad a condition to work with.
If you do want to removed the colour it's possible to sand the colour coat off and leave the original sanding sealer coat intact. Use a block to do this or you will rub through in patches. It's hard work though.
If you want any specific info then feel free to PM me and I can walk you through the process as you do it.
🙂👍