[andrewia]

The exchange could be limited to only existing, paid plans. New plans would require a device compatible with the new specification. I don't think it's too hard since American phone carriers were able to offer free LTE devices to users with activated 3g devices.

[whatevermang]

Correct. You need an existing plan, and clearly the carrier can see what handset you are/were using. It's also the right thing to do because they are imposing a financial burden on people. People with older handsets are (I assume) likely the ones that can't afford newer ones.