When configuring the Account Hierarchy, the documentation requires changing the Field Type from Master-Detail to Lookup. However, the Change Field Type button is not available. The user is unable to change the field type for Parent_Account_vod__c on the Child_Account_vod__c object.
If the Change Field Type button does not appear, it is an indication that the field is referenced by another process (Apex code, VisualForce Page, Workflow, etc.).
Review processes to determine the reference to Parent_Account_vod__c and remove it.
Salesforce Documentation: Notes on Changing Custom Field Types
CRM Help Documentation: Account Hierarchy