See the definition of ticket - "a valid document giving entitlement to transport, or something equivalent in paperless form, including electronic form, issued or authorised by the air carrier or its authorised agent"

"issued or authorised by the air carrier" - UA (via US award), correct?
"entitlement to transport" - on what? A flight, correct?

A ticket isn't an itinerary although a ticket may cover a series of flights comprising an itinerary, unless UA has started serving Australia-CDG.

Paper tickets used to have flight coupons - one for each flight in the itinerary. Paperless did away with the flight coupons, replacing them with a boarding pass for each flight. The EU rule covers IDB just as the U.S. rules do although the compensation is probably different. Would you expect IDB compensation for your entire itinerary if you were IDBed on the last segment of the return? If so, why in the world would you expect that? If not, how is the EU reg covering downgrades any different other than the penalty?

The OP has at least two factors against him in arguing what you suggest. US (and maybe *A) charged for international FC even when only one segment was actually flown in international FC - no compensation due. Arguing for compensation for the entire itinerary when only one segment varied in seating class is an uphill battle - most likely no compensation.

Jim