The approval needed to replace a Stage Plan with an Exception Plan must come from the Project Board. This is because the Exception Plan is created when a Stage Plan is forecast to exceed its tolerance levels, and the Project Board is responsible for approving such significant changes in the project's direction or management.
References: PRINCE2 Foundation official documentation and study materials.

One of the key responsibilities of the project board within PRINCE2 is to ensure effective communication with external stakeholders. This involves overseeing the project's strategic direction and ensuring that stakeholder needs, expectations, and communications are properly managed and addressed throughout the project lifecycle.References: = PRINCE2 7 Foundation handbook and online resources

The Project Plan in PRINCE2 identifies the management stages and timescales of a project. This plan provides an overview of how and when the project's objectives are to be achieved, by showing the major products, activities and resources required for the project, and structuring them within a timescale.
References: PRINCE2 Foundation manual, PRINCE2 official resources

Section: Volume A