Mister P. the information about not being able to re-credit Qcredits has been removed in this latest version!
Once you use Qcredits for an upgrade they are removed from your account. in exactly the same way that Qmiles are.. That is they are used even though you may not have flown the upgrade flight yet! Reward tickets (flights and upgrades) are transferable within one year of issue. That is the tickets and not how you paid for them (Qmiles/Qcredits)!