The top bearing of my FSA headset (Campy style integrated ACB) has been feeling pretty rough for the past couple of rides so I stripped everything down just now and the bearing is indeed pretty worn.

Am I right in thinking that these integrated headsets, being ‘sealed’ bearings, are one-shot jobs, non-serviceable, and require a new sealed bearing unit instead of a stripdown / regrease?

Just replace the sealed bearings,leave the rest as is.