I did the upgrade through CPW's website recently. Wanted to do it solely online to get cashback from Quidco. It's not a well laid out process because it looks just like a new contract purchase with no mention of upgrade, but if you go through to the end and checkout they will call you later in the day to confirm the upgrade.
Don't get concerned like me and call them after completing the online process because you still don't know it's been processed as an upgrade from the confirmation page and email. Salesman lied and told me he couldn't find the order in the system, so set up a new one on the phone so he could get the commission instead of me getting cashback. Only found out later on when I was called about the original order, which the first man had cancelled and replaced with his new one.
I could have cancelled the whole thing and started again at that point, but couldn't face it!