There are two questions here:
(1) should I migrate from Hibernate to JPA?
You don't need to, so don't do it now. Set up a Seam-managed Hibernate session, and you should find it easy to migrate your existing code. You won't need to change the existing entity classes, or make much change to existing data access code. You can always migrate to JPA later if you feel like it.
(2) should I migrate the business logic layer from POJOs to session beans?
Again, you don't need to, but since you will be doing the conversion to Seam components *anyway*, you may as well.