Just an update. I contacted the SV local agent in China to request a rebooking on QR flight and the agent submitted the request to SV. The next day I checked SV website and my entire itinerary has already been changed to QR. To make sure the ticket is valid I also checked the QR website and I can even select seats in advance. After reading all the complaints in here regarding SV customer service, I can't believe it's that easy to switch to QR. Guess I'm lucky.