Strangler Fig Pattern
When you find a monolithic application that is difficult to maintain or scale, you can use the Strangler Fig pattern to gradually identify parts of the monolith and replace them with new microservices.
As soon as your microservice is ready, you can start routing the traffic to it, while monitoring. Slowly, you do the same with other parts of the monolith, until you have replaced the entire monolith with microservices.
It is important to mention that monoliths feature needs to work, so this approach may implies to maintain data consistency across new and legacy systems.