According to the Project Management Body of Knowledge (PMBOK) guide, when a significant change is requested that was not part of the original plan, the project manager should first analyze the impact of this change on the project's scope, schedule, cost, and resources. This analysis should then be presented to the senior manager or project sponsor, who can make an informed decision about whether to proceed with the change.
This approach ensures that the implications of the change are fully understood before any action is taken, and it allows the project's stakeholders to make decisions based on the most accurate and up-to-date information.
This is a key aspect of effective change management in project management. References: PMBOK Guide, 6th Edition, Section 4.5: Perform Integrated Change Control.