I don't know.
I do see seats coming into the system in C. E.g. I checked May 1 and it shows C9.
I also note that the flight is currently a 757. It shows C inventory even though it's a 2-cabin aircraft because it presumably connects from 3-class international flights and that allows UA to sell C straight through.
Could be that the change of aircraft hasn't been reflected in fare class availability yet? Or could the change to a 777 not really be certain?
Once the 777 goes to a 763 for UA809, C inventory appears...
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View from the Wing: A blog about Free Miles and Free Markets