A new relevant project stakeholder suggested a change to some of the project parameters midway...

According to the PMBOK® Guide, integrated change control is the process of reviewing all change requests, approving changes, and managing changes to deliverables, organizational process assets, project documents, and the project management plan. It also involves communicating the decisions on the change requests to the relevant stakeholders. The project manager should evaluate the proposed change with integrated change control to determine its impact on the project objectives, scope, schedule, cost, quality, risk, and benefits. The project manager should also discuss the change with the project sponsor, who is the person or group that provides resources and support for the project and is accountable for enabling success. The project sponsor can help the project manager to obtain approval for the change, resolve conflicts, and align the change with the strategic objectives of the organization. References:

PMBOK® Guide, Seventh Edition, Section 8.2.2.1, “Integrated Change Control”

PMBOK® Guide, Seventh Edition, Section 4.4.3.2, “Project Sponsor”