A thorough suite of complex testing the system end to end will greatly help to ensure the project is a success. Never cut any corners on testing.

In my experience with complex systems upgrades, a proactive approach is essential. Establishing a cross-functional team early in the process, including representatives from IT, operations, and relevant business units, helps identify potential risks and ensures all aspects are considered. I also recommend a phased rollout instead of a "big bang" approach. This allows us to monitor the new system's impact and address issues in real time before they escalate. Additionally, regular training sessions for end-users can significantly reduce resistance to change and empower users to become advocates for the new system.

When dealing with a complex systems upgrade, determining the right risk mitigation strategies starts with a comprehensive risk assessment. Break down the project into its critical components and identify potential failure points. For example, during an electronics upgrade I managed, we ranked risks based on impact and likelihood, focusing on the areas most crucial to system functionality. Implement layered defenses, such as redundancy for critical systems and fallback protocols in case of failure. Continual reassessment throughout the upgrade ensures you adapt strategies to evolving risks, keeping the project on track while mitigating major disruptions.

The most important activity is to test the new system under realistic conditions, and avoid convenient test plans. The new system should be divided in blocks which could be tested to the extent of the technology, for instance scaling down the test currents to match the max range of a test set. However, reducing the test currents need to be acccompagned by maintaining the same source impedance, very often people test distance relay using a constant voltage which can lead to irrelevant or wrong test results.