Modern guns can demand more hull than an old destroyer has

Could a Fletcher-class destroyer replace its five original guns one-for-one with modern 5-inch/54 or 5-inch/62 mounts?

Not easily; the newer weapons would require extensive hull bracing, so a paper one-for-one replacement could overstrain the ship.

The proposal treats gun count and calibre as if mounts were interchangeable packages. The immediate constraint is the Fletcher-class hull rather than whether a newer weapon can fit in the same deck footprint. A modern 5-inch/54 or 5-inch/62 installation transmits loads that require substantial bracing. Refit feasibility depends on structural load paths and recoil forces as well as available space.