How far a ship class can change between hulls

How extreme can fixes to later ships become before they form a new class?

Very extreme; historical practice often kept heavily revised ships under the original class name.

A first-of-class ship often reveals defects only after construction or trials. Later hulls can receive extensive corrections while governments and navies continue to treat them as one class. Historical precedent permits very large differences before nomenclature catches up. The answer warns researchers not to assume every member of a named class was technically identical.