I'm really torn right now... on one hand, it's the WSJ. They speak volumes and don't usually open their mouths unless they're very sure of their sources.
On the other hand, I can't believe Apple would do this. There isn't a single iPhone 4 user out there that has completed their 2-year contract with AT&T or Verizon. They'd be pissing in the faces of every single iPhone 4 user by not allowing them to upgrade.
As much as I trust the WSJ, I'm calling BS. This just doesn't add up, and I don't think Apple would screw this many customers for a few extra bucks. IMO, they'd lose more in iPhone 5 sales than what they'd make from Sprint!