JPA is just a specification.In market there are many vendors which implements JPA. Different types of vendors implement JPA in different way. so different types of vendors provide different functionality so choose proper vendor based on your requirements.
If you are using Hibernate or any other vendors instead of JPA than you can not easily move to hibernate to EclipseLink or OpenJPA to Hibernate.But If you using JPA than you just have to change provide in persistence XML file.So migration is easily possible in JPA.
