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
Great Infograph on Frequent Flyer Traveling from the New York
NPD is reporting 85 percent of U.S. consumers say price