We have a new field, which is a dropdown, and the field is mandatory. So it will be filled in upon creating a new record (account). We would like to prevent that this field is modified later. We want to use this field to determine the territory an account belongs to, and because of API integrations it is essential that the territory does not change. 

How can we set that up? I can only find documentation about general write permissions for fields, but we need separate field permissions for creating and modifying. We have also tried to play around with coding some validation rules, but we can't seem to determine if a field value has changed, because we don't know how to retrieve the old value. Does anyone know how we can block field values from being modified? Thanks!