Why Japan replaced 7.9-inch guns with 8-inch guns

Why refit earlier Japanese cruisers from 7.9-inch to 8-inch guns when the difference was small?

The refit standardised logistics and supplied a slightly longer, more powerful gun while remaining legal under the treaty system.

Replacing 7.9-inch guns with 8-inch weapons standardised logistics across the cruiser force. The replacement gun was also longer and somewhat more powerful, so the change went beyond a nominal shift in calibre. Treaty rules permitted the 8-inch weapon. The navy could therefore combine a modest performance gain with simpler support arrangements.