It is possible, but not only do you need to book the flight before they expire, but also spend them.
From my experience, if that upgrade later on gets cancelled for any reason, the Qcredits will vanish. This could happen due to cancellations from either side or aircraft swaps. Generally QR is quite generous in these situations with rebookings though.
I once had that situation where I had a C->F upgrade and they swapped the A380 for a 777. When I called they allowed me to change to a later A380 and even change the connecting flight to a different airport.