Within NA, on widebodies (especially the B77W/L, YBYL is the common protocol. One time I was at the gate at YYZ and enquires about my upgrade (I was
#1 on the list), and I was told not to board with Z1 as they hadn't processed the eUp by the time boarding began.
The SOP may be that the GA would come onboard to upgrade whoever is next on the eUp list, but if they are up against their KPI of OTP vs following recommended SOP, most would opt for keeping with KPI.