G3s serve future needs rather than replace old ships one for one

Would four G3 battlecruisers replace the Royal Navy's five 12-inch battlecruisers on a one-for-one schedule?

No; the Royal Navy built new ships for anticipated future requirements, while older hulls could enter ordinary reserve or be sold according to whether they retained any use.

The G3 program answered the fleet the Admiralty expected to need next, not a bookkeeping requirement to retire a named predecessor. Older 12-inch ships could be placed in ordinary reserve if they retained mobilization value. Hulls judged useless could be sold without claiming that a particular G3 had replaced them. This distinction matters because British postwar manpower and procurement policy did not yet operate through the later one-for-one language used while rebuilding fleet numbers before the Second World War.