(my opinion)..
Verizon says it could have the 4th gen iPhone and if they do get it, it would be smart on AT&T to let 3g owners upgrade early and sign another 2 year contract. If AT&T didn't let 3g owners upgrade and they had to fulfill their 2 year contract, by the time the contract was up and Verizon had the 4th gen iPhone people would switch to get the iPhone like i switched to AT&T to get the phone. By allowing 3g owners to upgrade and sign another contract would mean another 2 years with AT&T, which means if Verizon gets the 4th gen iPhone you would still be locked in with AT&T.
Verizon says it could have the 4th gen iPhone and if they do get it, it would be smart on AT&T to let 3g owners upgrade early and sign another 2 year contract. If AT&T didn't let 3g owners upgrade and they had to fulfill their 2 year contract, by the time the contract was up and Verizon had the 4th gen iPhone people would switch to get the iPhone like i switched to AT&T to get the phone. By allowing 3g owners to upgrade and sign another contract would mean another 2 years with AT&T, which means if Verizon gets the 4th gen iPhone you would still be locked in with AT&T.