Why Britain planned large Spiteful and Seafang orders

Were the planned Spiteful and Seafang orders intended to replace early Seafires and other fighters?

Yes, and the large totals also reflected the aircraft reserves expected for an invasion of Japan.

Planned Spiteful and Seafang production was intended to replace earlier fighter types, including early Seafires in naval service. The scale of the order also reflected British planning for the invasion of Japan. A campaign of that intensity would consume aircraft through combat, accidents and sustained operations, so nominal squadron establishments were not enough. The figures therefore represent both modernization and the reserve depth needed for a demanding Pacific campaign.