Migrating legacy monolithic application to microservices is very complex and require more attention because Failure in one part can bring the whole system down. Higher database complexity, Improper resource utilization etc.
There are mainly two strategy to migrating monolithic application to microservices
1) Implement new features or functionality in a separate microservice.
2) Extract the new microservice from the existing Monolithic code base.
Read the full article Guide To Migrating A Monolithic Application to Microservices