Trying to get through to you here... o2 are not selling sim-free.
You think o2 will publish details of how to detract business from them? Yes you can upgrade with us... or not pay us a penny and buy an unlocked phone from Apple and use any sim you want! Yeah... that'd be a great business idea wouldn't it? They're not going to publish that as a possibility to the consumer, they don't publish that info for any handset. They're in the business of selling you subsidised handsets to tie you into their network with a contract.
Buy a phone from o2 with any subsidised price and it's an upgrade.
Changing a rolling out-of-contract monthly 2G/3G tariff to Simplicity 30d or 12m? That's an upgrade as you're agreeing to a new term.
Wanting to renew a term on a pre-existing 2G/3G tariff that's no longer offered? No can do, you've got to pick a new tariff from the list, which you can't buy until the 24th.. that's an upgrade.