CircleCI's blog post discusses the challenges and solutions involved in modifying their critical systems without causing downtime, particularly focusing on permission checks that ensure authorized actions on their platform. Initially, these checks were based on GitHub’s access model, but complexities arose from using two subsystems, leading to performance issues and vulnerabilities during API outages. To address this, CircleCI undertook a careful, phased migration process, consolidating checks into a single subsystem and eliminating reliance on an expensive "user profile" cache to enhance performance and reliability. The migration was conducted in stages, with extensive logging and metrics collection to ensure that new implementations matched or exceeded the performance and reliability of the old ones. CircleCI emphasizes the importance of testing in production, highlighting strategies such as using feature toggles for quick reversion and relying on comprehensive telemetry data to build confidence in system changes. This approach allowed the team to iterate rapidly and address unforeseen issues without impacting customer experience, demonstrating the value of cautious and informed development practices.