Originally Posted by findark

I believe it was always split at check-in, not T-24. The sweep running at T-(24+ε) just tagged the PNR with the appropriate WLPx SSR code which notified the check-in code to split the PNR and process the addition to the upgrade standby list at the corresponding priority. The check-in interface has an option to decline this for CPU priority but nobody every added it for SSR-driven upgrades. Technically you could still check in without the split, but you would need to find an agent who knows what an SSR code is (and you the pax probably need to be able to tell them that it's an SSR issue) and how to remove it from the PNR manually before processing the check-in. If you managed to do this, you would not be added to the upgrade standby list (though theoretically could still clear off of the PZ waitlist process prior to gate control).