Here is an old list. Mostly still good:

Fare Domestic Upg. Pecking Order
F First Class 1
J NA 1
C NA 2
P Discounted First 2
A Discounted First 3
D Discounted Business 3
G First Class 4
I Discounted Business 4
Z Discounted Business 5
B Yes 6
Y Yes* 6
M Yes 7
S Yes* 8
H Yes 9
Q Yes* 10
K Yes 11
L Yes* 12
U Yes* 13
T Yes* 14
X Yes* 15
V Yes* 16
E No 17
N See Notes NA
O NA NA
R Booked Award NA
W Premium NA
W+ Comfort+ NA