Since doing so would mean the mapping xml would have to change to match the new structure and classes would need new properties and getters/setters, I'm guessing it is not supported. I have not found any specific information about this, but it appears Hibernate is built for running queries against a DB not changing the structure of the DB. Is that right, or have I missed something?

you're right, Hibernate expects the db structure to be fixed. It provides the capability to store & retrieve Java objects to/from a relational db without having to write any SQL, and to be able to switch between a (large) variety of databases with no change to your application code.

Bull. Use <property name="hbm2ddl.auto">update</property> but read the docs and realize the limitations of automatic DB schema evolution first.

i read the docs, and it says "update" is a valid value for hbm2ddl.auto, but doesn't say what it does. One could infer it will update the db to match the existing *.hbm.xml files. But this only happens at SessionFactory build time, and presumes one has already updated everything else appropriately. It certainly doesn't support on-the-fly db changes, which is how i interpreted the original question.

What is an "on the fly" database schema update?