I was told on the phone that this service is guaranteed. So I’m putting together what other people have received and assuming that the timeframe given 2 weeks prior to your travel date gives them enough time to process the upgrade, even if that means someone has to wait 48,72 hours to hear back from them. They will process your upgrade request and then either give you the option to collect in person(which considering the ordeal i would go in person) or to post via the couriers. Im waiting for the 2nd May to apply for my upgrade