The upgrade booking codes are as follows:

FC: X (NF)
BC: I (NC)

The award booking codes are as follows:

FC: R (XF)
BC: O (XC)
EC: N (XY)

(The two-letter booking class designators are internal to UA and show up on the tickets, boarding passes, etc).

Thank you, KiwiFlyer for this long-overdue D-check on this thread.