I just booked a return PVG-NYC, via SFO, overnight, with a single SWU. When I called to do the upgrade, I simply pointed-out that the trip involved a less-than-24-hour connection, therefore, I was using a single SWU for the entire trip. The agent concurred that I was within the rules, and upgraded the full itin. At that point, I asked them to document the record that 1 SWU covered the one-way trip. Done.
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-- Bob