Originally Posted by salsarocks

Data point on this. I cancelled the 2 U seats and I booked 2 PE (P fare class) seats. Having checked that 2 U were available, I called them to upgrade from P to U but it appears that while they were in the process of reserving the seats for the upgrade, the 2 U seats (that were there before) disappeared, so I guess that either this "workaround" doesn't work or the "I" inventory was full or that "I" is only allowed for cash bookings