Questions Subaccount Import Scenario

We've added a segment to the subaccount in our TEST enviro.  I downloaded the list of existing subaccounts and I want to add the new segment values and re-upload.

Has anyone had any luck with this?  It seems to just ADD a new row, instead of updating the existing ID.   Pretty new to Import Scenarios so any resources on that would be helpful.

Here is my Scenario:

And, when I selected 1 record to process, which was the Second row (sub id 5) "000-000-100-"

it simply added the row below it with the additional segment....to make a new row (sub id 1881) "000-000-100-00000"

Thanks!

k2