According to the PMI Risk Management Professional (PMI-RMP) Reference Materials, the project management plan is the document that describes how the project will be executed, monitored, and controlled. It integrates and consolidates all the subsidiary plans and baselines from the project management processes1. The project management plan should be updated whenever there are changes in the project scope, schedule, cost, quality, resources, communications, risks, procurements, or stakeholder engagement2. In this case, the project team has decided to log the opportunities in the current project's risk register, which is a component of the project management plan. Opportunities are positive risks that may have a beneficial effect on the project objectives, such as cost savings, schedule acceleration, or quality improvement3. Therefore, the project team should update the project management plan to ensure the results of the opportunities are captured and reflected in the relevant subsidiary plans and baselines. For example, if an opportunity leads to a cost saving, the project team should update the cost management plan and the cost baseline accordingly.
References: 1: PMI, A Guide to the Project Management Body of Knowledge (PMBOKGuide), Sixth Edition, 2017, p. 89 2: PMI, A Guide to the Project Management Body of Knowledge (PMBOKGuide), Sixth Edition, 2017, p. 123 3: PMI, A Guide to the Project Management Body of Knowledge (PMBOK® Guide), Sixth Edition, 2017, p. 397

An Ishikawa diagram (also known as a fishbone or cause-and-effect diagram) is a tool used to identify and analyze the root causes and sources of a risk. It helps the risk manager gain a deeper understanding of the risk source and likelihood. (Reference: PMBOK Guide, 6th Edition, p. 139) An Ishikawa diagram, also known as a fishbone diagram or a cause-and-effect diagram, is a tool that can help the risk manager to analyze the root causes of a risk and to identify the factors that influence its occurrence and impact. An Ishikawa diagram can also help to visualize the relationships among different causes and to prioritize the most significant ones. By developing and employing an Ishikawa diagram, the risk manager can gain a deeper understanding of the source and likelihood of the risk and plan appropriate responses accordingly. Reference: The Standard for Risk Management in Portfolios, Programs, and Projects, page 72; PMBOK Guide, 6th edition, page 398.