I am a lowly P and so I did not get the SSWU, the way I read the above (and other great information here) and I also confirmed by calling AC (prior to purchasing), was that on my flight YEG-LHR 2 days prior I could request the upgrade on a L fare. Since the flight looked fairly open in terms of I (it was I9, for other weeks on the same day). So I thought I would take a chance on aerolotto (the return is U and it is already upgraded) for this flight.
So did I totally misinterpret this and I am out of luck trying to upgrade a L fare using an SWU 2 days prior to departure or even at the gate? Or did I read right the first time? The flight departs in a few days (Feb 7) and is still showing I9.
SWU cannot be used for L fares no matter what.
(if a kindly agent takes the cert and upgrades you anyway, that would be pure luck and outside the published rules.)