I did so with 2 airaves on my account via Sprint.com Chat.
I made no mention of the current devices, simply i wanted to use the upgrade on line x & line Y and told them the model phone's i wanted.
shipped to my door. My $0.00 per month airaves, are now in a 2yr contract.... well, actually, 1year, since i did this May 2011.
Again, only Sprint can actually do this. If you plug the numbers into any 3rd party vendor, it will error.