According to the PMBOK Guide, a project manager should put any new requirements in the backlog and submit a formal change request for approval, when a change in project scope is requested by a stakeholder.
This is part of the perform integrated change control process, which involves reviewing and approving all change requests that affect the project scope, schedule, cost, quality, resources, communications, risks, procurements, or stakeholder engagement. A formal change request is a document that describes the proposed change, its justification, its impact, and its alternatives. A formal change request should be submitted to the change control board (CCB), which is a group of stakeholders authorized to approve or reject changes. The CCB should evaluate the change request based on the project objectives, the project management plan, the project documents, and the stakeholder expectations. The CCB should also consider the potential benefits and drawbacks of the change, as well as its alignment with the project charter and the business case. The CCB should then communicate the decision to the project manager and the relevant stakeholders, and update the project documents and the project management plan accordingly. This process ensures a consistent and transparent approach to managing changes, and prevents scope creep or unauthorized changes that could negatively affect the project performance or quality. References: PMBOK Guide, 6th edition, pages 115-116,
121-122.