When trying to delete a Sales Territory used before, the user will be given the following notice: This record cannot be deleted because it has dependent records.
This is a default behavior of NetSuite. Sales Territory that had been already used cannot be deleted. Even with a Mass Update performed on all records that use the Sales Territory, the System Notes generated (old value of territory set to new value) become the dependent record in this case.
The solution is to set the Sales Territory to Inactive.
This can be performed by following steps:
1. Navigate to Setup > Sales > Sales Territories
2. Click on Sales Territory to be deleted
3. Click Edit
4. Inactive: Enter Checkmark
5. Click Save