Why missile-era alternate warships are harder to design

Why are pre-missile warships easier to redesign in alternate history than missile-age ships?

Because missile weapons are deeply integrated with sensors, command systems, power, and controls across the ship, while gun mountings are comparatively interchangeable and better documented.

Pre-missile warships are easier to redesign in alternate history because their equipment is better documented and their guns are comparatively interchangeable. A gun mounting forms a mostly vertical installation with control links back to the firing computer. Missile systems connect to radar, the combat information center, fire control and power services distributed around the vessel. Substituting a missile system can therefore require changes across much of the ship, while classified details make those changes harder to reconstruct.