Or they could be even less onerous and just offer a higher reimbursement rate for newer phones. Movie studios do something like this with theaters to encourage keeping movies playing - the longer it plays the higher the cut for the theater. This would work the other way around - the newer the OS the better the cut. That encourages carriers to both sell newer phones and update older ones.
Maybe work out the change so that based on current numbers the carriers make the same amount of money so they won't complain, but they now have incentive to make even more.