(Mods: Feel free to move this to another thread. I couldn't determine ideal thread to put this in)

I'm curious as to how United's ticketing system/pricing handles the following situation

A. Booked Flight A in E+ (using FFC, if that matters)
B. Took advantage of a cheap upgrade offer $B to upgrade the flight (Shows on Receipt as "Premium Cabin Upgrade")
C. Today, I needed to reschedule to an earlier flight. Kept F class, but had a fare differential of $C.

Current flight receipt shows new fare along with the "Premium Cabin Upgrade" line item. In a situation like this, does the fare differential cover the F class difference between Flight A and B...or Y class with the upgrade overlaid on top?