See the database section of the installation guide for guidance on the difference between the supported backends.
In 1.4 a script has been provided to enable users to migrate between the supported backends for a given version of OpenDNSSEC. The script is located here:
In 1.4 the supported backends are SQLite and MySQL.
You must create the MySQL database and database user manually before converting to MySQL.
If users wish to convert to another OpenDNSSEC version you need to migrate existing database to that version first - see Migrating from earlier versions of OpenDNSSEC.
Also see the MIGRATION file in the source code.