Is it possible to update an account segment in the PO charge string for already loaded POs?

Summary: We recently converted to Fusion on 7/1 and there are around 60 Purchase Orders we need to update one of the segments in the charge string. I have been trying to update this via the FBDI template but not having any luck. I either receive errors due to missing fields, or when I try uploading the full template with ONLY the updated field as being the only thing that changed, it adds a new line to the PO.

Can someone advise what I am doing wrong? See attached file with screen shots

The PO_Distribution_Interface tab in column M in yellow is the only field I need to update. The PO Header Action I have is UPDATE, and the PO Lines is ADD (this is the only option).