Are you changing to an iPhone? If so you would have to phone customer service.
I ask this because migrating a number from PAYG to pay monthly, doesn't need a PAC (when you phone CS they wouldn't give you one). And in the case of PAYG to PAYG you just put your old SIM in and phone to get the upgrade credit. Therefore you wouldn't have been able to use the form online.
Track usage, upgrade and get support with the My O2 appDownload My O2