Why Japan keeps the Kongōs instead of converting them

Why not convert or replace the Kongō-class ships under the separate battlecruiser allowance?

They were too new to replace legally, still fitted Japanese scouting doctrine, and gave Japan four useful ships where the same tonnage might buy only three larger ones.

The Kongōs are not intended to trade blows with American battlecruisers on equal terms. Their speed makes them reconnaissance and forward-support assets for the fleet. Replacing four existing hulls with three larger ships would cost scarce industrial capacity while reducing the number of available scouts. Both the Kongōs and Lexingtons remain vulnerable to each other's heavy shells, so the larger American gun does not erase the Japanese ships' operational value.