I am not referring to the upload, I am referring to the Update record server behavior.
There is the standard Update record behviaopr that is included with DW (this is the one you are using)
and there is an update record behavior for Data Assist.
open the server behaviors panel, click the plus button and go to:
WebAssist -> Data Assist -> Update Record -> Update Single Record
this is the update behavior you need to be using instead of the standard update behavior.