Originally Posted by
bocastephen
That mistake is on United - they could have used another fare class for these fares, or restricted the upgrades to Y or M and re-used B for alternate fares.
Likely the uniform fare lettering for economy across *A created barriers in this exercise. Something had to give and it was Instant Upgrade. Not defending UA, UA has been adding new options over the years and in their wisdom it was time for Instant Upgrade to go, so it could offer these other options.
At least it is not what DL has done with its WN-like fare structure.