Cannot change service user's "Display Name" propery in CCP for subscription with Exchange Shared Domains feature, an error "This field is mandatory" is shown for service user login field, however it is not possible to modify it.
The issue is considered to be a software issue with ID POA-91123, since the Exchange Shared Domains feature enabled it is not possible to change service user login. But due to the mentioned software issue the service user's login is not displayed and the validation fails.
The issue affects CCP only, it is possible to change service user's Display Name property via MyCP, e.g. by logging in as affected service's user.