Sub to parent converted server mysql_user remain parent => Failed to modify server : A MySQL user named <parent> already exists

After converting a subserver to parent with a new administration user name, and enabling the MySQL feature, the following error message appears:

Failed to modify server : A MySQL user named [oldparent] already exists

I've checked the new parent server's config file at /etc/webmin/virtual-server/domain/[int] and found that the mysql_user variable remained [oldparent]

After fixing this variable, it allows to enable the MySQL feature.