Just to provide the latest info as I spoke with a CSO about this exact issue today. He said Qcredits are non-refundable, as experienced by @
spec119. So if you use them once to upgrade a ticket and then need to change that ticket to a new date, you've lost your Qcredits, regardless of their expiry date! How useless is that?! QR's rules are so strict around this. Thankfully I've never had to change a Qcredits upgraded flight of mine. I just booked one today (SIN-JFK) for next summer - fingers crossed!