I added a couple links to the wiki (that I just created), but (emphasis mine):
25. Mixed cabin itineraries
For Flight Rewards booked across different classes of service:
- Flight Reward pricing is based on the highest class of service in the itinerary, per direction of travel.
- Changes to Flight Rewards booked with mixed classes of service, including upgrading flight segments booked in a lower class of service, are subject to availability and may require the additional use of Points, whether in whole or in combination with cash.
- Courtesy upgrade requests are not permitted, including on day of travel at the airport.
That's the closest thing in the terms I can find that would apply to this, but I don't like the phrasing "upgrade requests".