Change of Mind

I upgraded my phone this morning, was looking for more information about data contracts and was told I could do an add on after I upgraded my contract. Looked into it and realised that the add on was at normal price and if I changed my plan to a better one I could get a better rate on data so I called back this afternoon to say I had changed my mind and would like to change my plan. I did not want to change the phone just the plan to a more expensive one and was told that I couldn't do it. I was told I had to wait for them to deliver the phone and tell them I didn't want it and send it back. I then had to wait for them to cancel my contract and then sign up for a new one. I don't understand what the problem is, I just want a different plan, a more expensive one at that and I changed my mind in a few hours. Surely this should be able to be solved via their computer system?