Why replace existing shell stocks?

Why would Britain replace the old shells instead of continuing to use its stocks?

Because their defects have been demonstrated. Existing stocks would be replaced once they were known to be ineffective.

The case for using existing stocks depends on believing the older shells are effective. Once their failures have been demonstrated, the Royal Navy has a reason to replace them throughout the fleet. The counterfactual allows eighteen months for that changeover as a conservative estimate. The historical replacement programme achieved much of the change within about twelve months.