Changing the default format in your /etc/my.cnf file should only apply to new tables, and does not erase databases or tables. The configuration option is documented at:
It's important to review the documentation because there are some instances (e.g. MySQL 5.1) where using this file format could make your databases inaccessible. Also, if you plan to switch over to MariaDB, you may want to continue using the Antelope file format for InnoDB if you use replication functionality: