How the GUPPY program reshaped surviving American fleet boats

Did the United States run a postwar submarine conversion program comparable to British T-class changes?

Yes. The GUPPY program refitted usable wartime fleet boats extensively and helped shape later American anti-submarine doctrine and force structure.

GUPPY modernized late-war fleet boats instead of discarding a large, still-useful conventional-submarine inventory. The United States built relatively few new diesel submarines after Nautilus because nuclear boats became the preferred development path. Refitted wartime hulls therefore formed a practical bridge and were passed to allied navies in considerable numbers. The comparison with Britain's postwar T-class changes and O-class replacements shows two different ways to move beyond wartime submarine designs.