I have asked same question of good bodymen. some will say always take to bare metal and go from there, others have said, ( and I have found this to work for me), IF what is already on there, and there is NO rust under it and it will not come off ever, what is already there can work as a base. .
I have had some cars with OEM paint that I could DA off the paint (topcoat), with 80 grit, BUT wow that primer was one there SO good and NO rust under it at all. I think in that case it can work.
BUT if there is ANy sort of rust under the old primer, it needs to come off and have clean metal.
like said, it can depend on what you want. and what you will spend on materials.
like said, air craft stripper can really get rid of paint, but if you have more layers than OEM, it can take a lot and it can take a lot of paper and air and time to da IT ALL OFF. any paper coarser than 80 grit will leave a problem with scratches.
as you have heard, your finish will be no better than your prep. sad but true!