I am currently performing that migration.
During the migration process I have EJB2.1 and EJB3 in the same application. I don't plan to put this 2 ejb types together in production. However, it should work fine.
Migrating only session bean is very simple and work quite fine. However, you will have to put these ejb3 in a separate jar file that your EJB2.1.
When you will migrate you entities, the only problem I see in having both type of EJB is that I don't think you can have managed relationship between EJB2.1 entity and EJB3 entity. Otherwise migrating is going well. To accelerate a little bit the migration process, I have use xslt files created by oracle.
You can find this xslt files at the following location:
Hope this will help,
An Phong Do