Originally Posted by
WineCountryUA
My understanding is a supported upgrade when split will maintain the original elite's status but may fall to the bottom of the status window. But this is an area of fuzzy understanding and there have been some conflicting reports if it maintains the fare class priority -- but no doubt the status is maintained on the segments after splitting.
I think there is some sort of distinction between "split immediately prior to check-in" and "split during check-in". Flying LHR-[US], since I don't hold a US passport, I check in with an agent; when I'm with a companion and we're both WLd, so a split is going to happen, and the split happens ex-LHR, my companion reverts to BG4 rather than BG1 and gets no precheck, which suggests the system saw the companion as split
before check-in begun.
By contrast, flying US-anywhere else, when the res is split my companions retain BG1 and precheck, which suggests the system "remembers"/inherits my identity/status during the split.
No idea what a human agent is doing differently than an app, but there does seem to be some sort of distinction there.