On grade export - XML file we have a option "Export new or update grades only. This feature works as described, however need few UI changes to make it more clear to moodle users.
In order to get the expect result, the user must search for gradeexport and enable the XML file option. However, there is one issue with this export tracking. It converts something that you would expect to be a read-only operation (exporting grades) into a write. If you want to ensure that every update gets sent, then you have to make sure you never lose an export file, or you will lose some changes.

See MDL-44923.