I gave up - I called United and they were perplexed. The 1K agent put me on hold and came back and said "it looks like even though the inventory is there, the fare is not eligible for transatlantic business class unless you route through London, but we no longer serve Paris-London since BMI pulled the route. You could route CDG-FRA/MUC-LHR-SFO, otherwise you would need to purchase a D fare."
I've booked the ticket with AA/Iberia at some inconvenience - SFO-JFK-MAD-FCO outbound and Paris/Orly-MAD-ORD-SFO on the return, although the fare was fantastic: 2625 plus taxes, all confirmed in F/J. United wanted 3400 for similar SFO-FRA-FCO/CDG-FRA-SFO and would not confirm the C class seats. Bah.
I wonder how many confirmed Y>C upgraders are on the flight that the OP wanted. It's a shame that UA denies paid discounted C travel on a flight that will probably have several people upgrading to C from Y with SWU's.