That's not absolutely the case....
If you upgrade your phone they can require you to change your plan. Specifically they can make certain phone require a specific plan at a minimum... they simply would make the specific plan be the non unlimited.'
However they can't make you change your plan while you're under contract if you keep the same phone and want to keep the same plan ... otherwise you'd be able to get out of your contract free.
This.
It's gonna be epic fail when I decide to get a new phone :/

makes me sad since sprint and t-mobile say they are staying away from this. if this happens then how will it effect tethering -- does anyone know what an 'average' person uses in data from web browsing when tethering? (just curious)