If you want to move the databases from one user, to another user, you can do so with the following command:
VERBOSE=1 DBUSER="da_admin" DBPASS="da_adminpass" USERNAME="username" NEWUSERNAME="newusername" /usr/local/bin/php /usr/local/directadmin/scripts/change_database_username.php
Where you can grab the da_adminpass from /usr/local/directadmin/conf/mysql.conf, and of course, replace all bold info with the appropriate info.