Hi Steve, I had a look at the XSD you pointed me to.

You seem to go for approach 2) now. I may be miss-led by the many comment differences, but as it seems many of the types from the custom XSD are not too different (or even unmodified) to the corresponding ones in orm.xsd, so approach 1) may actually be more feasible. My concern about 2) is that it makes it more difficult to incorporate upstream changes (say in JPA 2.2). It's easy to "forget" new elements added to existing types, whereas you'd get those automatically when extending the original types.

Regarding the concern about migration, I'm not sure I get what you mean. AFAICS an instance document needs to reference the Hibernate namespace, how could it otherwise make use of the extended types?