Where I live you can upgrade without a contract or with one. I believe you have to be out of contract in order to upgrade without a new contract though. If you are out and you can't upgrade without resigning it might be their terms where you live. There's no "see no contract prices" button when trying to upgrade?
Go to the upgrade center, go to the bottom and click on "customize your own bundle" or something like that. You should be able to then change between contract and non contract offers. If not then like I said, its probably where you are.
Cell phones are the same way. Upgrade to a new phone, have to resign a new contract.