Why late-war upgrades mattered more than a one-for-one ship swap

Would replacing late-war vessels one-for-one with their pre-war versions improve their performance?

Probably not, because late-war radar and other upgrades were important parts of the ships' combat value.

Ship identity matters less than the vessel's technological state at a given date. A one-for-one substitution would probably perform worse if the earlier vessel lacked its later upgrades. Radar became increasingly important as the war proceeded. Sensors, weapons, and refits determine what a ship can actually do more precisely than the hull name alone.