Travelling on an S-class E+ ticket ORD-LHR this evening (UA 938 dep 21:21). OLCI last night, no chance to pay the $649 to upgrade to C/J

What determines if you can do this? Availability is showing F8 P8 A6 C9 D9 Z9 Y9 B9 M9 H9 Q9 V9 W4 T0 S0 K0 L0 G0.

Thankfully I have 21J (best seat in the house IMHO) but still wanted to u/g.

What about my return flight on Sunday (UA 959 LHR-ORD dep 15:40)? Will I get the buy-up then from Mr Chicken? (OLCI not available outside US)

Availability for that is F6 P6 A5 C9 D9 Z9 Y9 B9 M9 H9 Q9 V9 W9 T9 S9 K9 L9 G0.