Having done this kind of migration/rewrite multiple times, the way you succeed is starting with acceptance tests that verify functionality from the API layer th

Having done this kind of migration/rewrite multiple times, the way you succeed is starting with acceptance tests that verify functionality from the API layer that is implementation agnostic. After the tests are in place, you break off small portions into microservises and ensure tests pass. You pass a small percentage of traffic through the new arch, fixing bugs and leveraging telemetry. You eventually slide all traffic to the new architecture. You want to get something receiving traffic asap as to start getting feedback. Often this means taking something smaller or simpler out of the lagacy codebase first. Doing a complete, side by side replica is a recipe for disaster. Think MVP. We've even introduced traffic routing based on feature sets so you can route users who don't use edge case features the new arch which has yet to incorporate the those features while keeping others on the old arch.