They explain what happens in order to pay off the phone when you want a new one. There doesn't seem to be an explanation of what happens once you have done that, and actually got your new shiny upgraded phone. Are you engaged in a new 24 months contract for just the phone, or both the phone and airtime. I don't think it's a bad thing as such, but it would be nice if they made it clear.
One of the better benefits I see of this is that if you keep your phone and airtime for the full 24 months, the phone is paid off, and rather than having to try and jump ship asap or pick something you don't want, it defaults to just the airtime, which is nice. No more having to pay silly amounts after the contract has expired.