I don't know if that's strictly true, Mike.
From what I can glean from the Iwata Eclipse parts catalogue the BCS (siphon), SBS (side feed), BS (small cup) and CS (large cup) could share the same head assembly, air valve and needle assembly. To convert the BCS to a gravity fed you would need only the 6-CS Body. That would continue to use the .5mm needle and head assembly. It's probably a good idea to replace the washer, o-rings and teflon needle packing set.
However, in order to convert it entirely to a CS you would need to add the .35mm needle, nozzle cap, nozzle, lid, air valve spring, needle spring, and spring guide, as well as replace the packing head washer and o-ring and teflon needle packing set. All of which would probably cost more than a new CS. New, fully assembled airbrushes typically being less expensive than the sum of all their parts!
With a Badger 100/150 series AB it's much simpler as all the parts except for the body are the same. I could buy a 100G body and convert my 150 quite easily. Not much point, though.