If you don't have any sql native request in the application (that means that you are using just HQL, Criteria queries) then to move from a database to another you just need to change the driver database and then in the hibernate config file to replace the: url, hibernate.dialec and the jdb driver className.
Of course you need to have a similar arhitecture in both databases (same tables, same sequences (the hibernate_sequence is usually used), same stored procedures etc.).
I’ve looked at a lot of different solutions, and in my humble opinion Aspose is the way to go. Here’s the link: http://aspose.com
subject: Required HELP in Hibernate Conversion, SQL Server to Oracle