Is it possible to change the existing Employee Role Hierarchy?

Yes, You can change the existing employee roles from employee directory in two ways:

1.One by One
Go to Employee Directory > Action > Edit > Access Details > Employee Roles
> Select the required field

Employee Directory > Bulk Edit > Access Details > Employee Role >Enter the required role

To view various roles and their respective access in the system, Select “View Roles” in Employee Directory on Web Dashboard