The Thunderbirds flew T-38s from about 1974 (reflecting the need to conserve fuel after the OPEC oil embargo) until 1982. They switched to the F-16 after a crash during training that killed 4 pilots. They were doing a formation loop at low altitude when the leader's control system failed. He went straight into the ground and so did the other 3 pilots, all in a nice neat row.

They had a fairly nice show with the T-38s but the plane just isn't as well suited for the role as a fighter like the F-16. The T-38 has a much lower thrust to weight ratio and just can't handle vertical and high-g maneuvers as well. They flew the F-16A from 1983 to 1991, then switched to the F-16C in 1992. From what I've heard, their planes are fully combat ready, only needing a quick paint job in the event they're needed in action.