In order to book an upgradable fare on United you have to buy a ticket in fare class W or higher. Here are the fare classes above W.
V
Q
H
U
E
M
B
Y
Only airlines could convolute something so simple as the ordering of the alphabet.
Related
The following was published in Dealerscope Magazine in December 2010:
Reid Hoffman posted some reflections on Greylock’s investment in Airbnb.