Azure Logic Apps Consumption Integration Accounts were powered by approximately 60,000 Azure Functions apps running on older runtimes. The team successfully migrated all these applications to the newer Functions v4 runtime without requiring any customer action. This was achieved through a meticulous, multi-stage approach designed to ensure compatibility and minimize disruption.The core strategy involved shadowing, where real production traffic was routed to both the old and new runtimes concurrently. This allowed for direct comparison of every single result without the unproven path affecting customers. A key aspect was establishing a robust parity bar, ensuring that 100% of eligible traffic was analyzed to differentiate genuine bugs from inherently nondeterministic workloads.Any real divergences detected were meticulously fixed before any traffic was shifted. The rollout was progressive and reversible, meaning traffic was gradually moved region by region based on traffic hashing. A crucial feature was the ability to roll back the entire migration through a simple configuration change, taking effect within minutes.The retirement of the old applications was handled with extreme care. The old apps were stopped first, followed by a significant observation period before they were permanently deleted. This phased approach ensured that any unforeseen issues could be addressed without impacting customers.This complex migration was possible because Microsoft owned and operated the underlying compute infrastructure. The durable customer data, such as agreements and schemas, remained untouched, and the actions performed were pure transforms. This control plane allowed for parallel execution and comparison of both runtimes.The primary challenge was not the new runtime itself, but proving its compatibility with the existing customer workloads. The legacy runtimes were nearing end-of-life, posing increasing risks. The move to Azure Functions v4, with its isolated worker model, represented a significant architectural shift.Standard lighter-weight migration options like in-place upgrades or simple deployment slots were deemed insufficient due to the host model change and the scale of the operation across 60,000 applications. The migration ensured no customer-visible disruption, although a small number of customers encountered a brief edge case under heavy load before a rollback was initiated. The team absorbed the complexity to provide a seamless experience for their customers.