Figured it out.
On this headset, the retainer is beveled and expands when fitted down into the upper cone. There's a tiny space beneath the upper cone; I slid a flat screwdriver tip into it and pried up very, very gently. That did the trick.
FSA Orbits seem pretty crappy. I've had my IRO about 3 months, put maybe 300 miles on it, only a few days in rain. Already it looks like there's some corrosion in the cups.