I'm wondering if there is any way I can get this to work. One thought I had was to book AAA-BBB-CCC all in XN at the saver economy price and then call to try to switch the AAA-BBB segment to IN and pay the mileage difference to saver business. But I expect that they would need to reprice the whole journey and XN would need to show available for the connection. Is that right?
Two options book the longer connection and hope the shorter connection opens later, or book XN and try to find someone who will be willing to waitlist you for IN. Since IN is alreadyy open on the long flight, that should clear immediately and get you the flight combination that you want. It's just, you may spend quite a while trying to find someone who will actually waitlist an award these days.
What you're seeing is married segment inventory at work: for whatever reason, UA has decided that the earlier flight should only be sold as part of certain connecting itineraries, and AAA is included. The problem is that UA doesn't marry segments across cabins, so while that opens XN on AAA-BBB-CCC, it does not make the IN/XN combination available, because that would require XN on BBB-CCC as a standalone segment, which isn't currently available.