If you were going to Upgrade a contract over the phone (as an existing customer) you would ring up, give them your password, and then order the phone/set up a new contract/upgrade to a new term (whatever you want to call it).
If you couldnt phone up (cuz O2 arent offering it) and someone else went in for you, took all the relevant details, and then during putting the contract through (in store) you rang the person who the contract was for to confirm they wanted it obviously, can you see any issues?
Basically, the GF cant make it into a store tomoro (she works in a school.)
I am going to order my 4 in store, so is there any reason I cant upgrade hers too? Logically, I cant see a problem.