My recent expriences have been different. BOS-SFO-NRT-BKK for November travel, using a SWU, BOS-SFO waitlisted Y-->F, other segments cleared previously or at ticketing. I received both an SMS and an email when BOS-SFO cleared. However I did not call in for a seat assignment for 7 days. I still had my upgrade, so no problems.

I know many people have mentioned this situation, possibly losing an upgrade by not calling to confirm within 24 hours, but I have never experienced it.