Which battleships should receive new 15-inch guns first?

In what order should First World War battleships receive improved 15-inch guns?

Start with the ships planned for the longest remaining service life, because they will benefit from the improved guns for the greatest time.

Modernization resources produce the greatest return when installed in hulls with many useful years remaining. Long-retained ships should therefore receive priority, with the comparatively manageable change from a 42- to 45-caliber 15-inch weapon used where feasible.