Mapping Parent Account to Case entity

Hi experts

I have an out of the field in the account record called "Parent Account". I have also created another "Parent Account" field in the case record so that when the user chooses the customer, they can also see the parent account. 

For this requirement, I would like to map the parent account field in account entity to the parent account field in the case entity. But when I create the field the system creates a relationship itself and doesnt let me change it. 

How can I change the mapping? Or is there another approach to take for this?

Thanks,

Jon