The upgrade offer is only applicable to the flight(s), of which the flight number(s) is/are indicated in the email announcing the offer. It may not be applied to any other flights in the same or another booking. Therefore, you may change your travel date if doing so is in accordance with the fare rules and conditions of the original ticket purchased. The appropriate fare difference(s) and date change fee penalties will apply.
Your flight will also be rebooked, but your seat will not be reserved in the class of travel you that upgraded to. In addition, neither the upgrade fare nor any Qmiles spent will be refunded.
Furthermore, the upgrade offer is not transferable to other passengers. It is only applicable to all passengers in the booking reference, indicated in the email announcing the offer.