Using the XML Backup/Restore feature for migrating between database types (or from HSQLDB) is not always feasible, especially for large instances. This process would benefit from working similarly to the migration process in FISHEYE as explained at https://confluence.atlassian.com/display/FISHEYE/Migrating+to+an+external+database.
Since this would be a new feature to Confluence, it might also be helpful to add functionality to this process to migrate from one database type to another. This would improve the process for both customers and support, by decreasing the time it takes to perform the migration as well protect against data corruption that can happen in the current process.
If it's feasible to do a leaf-to-root, enumerate-and-progressively-output approach, it could also lead to filesystem backups that don't have the current limitations on site size.