When faced with a significant change that impacts the project scope or environment, such as the termination of operating systems, the first action should be to engage with stakeholders to assess the impact. This is in line with the agile principle of collaboration and early stakeholder engagement. By scheduling a session with stakeholders, the project manager can gather insights on the implications of the change, which is essential for informed decision-making and planning. This approach is supported by the PMBOKGuide, which emphasizes the importance of stakeholder engagement and communication when dealing with project changes.
Additionally, the Professional in Business Analysis (PBA) materials highlight the need for business analysis to understand the requirements and constraints of the stakeholders, which is critical in this scenario to determine the next steps for the project.
References: PMBOKGuide and Professional in Business Analysis (PBA) Reference Materials.