Keep the earlier edition.
A replacement does not erase the instruction or decision that came before it. The file should make the change visible while preserving the history.
Ask what the change affects.
A new destination does not inherit an earlier destination decision. Useful unrelated work may remain useful, but that is a question of applicability—not a reason to mark everything current.
Preserve the open question.
A newer attachment may be incomplete or inconsistent. “New” and “authoritative” do not mean the same thing. The office still needs the basis required by its procedure.
Give the next person context.
A covering colleague needs the current proposal, the affected earlier work, the unresolved issue, and the next permitted act. They should not have to compare every inbox attachment to discover what changed.
A product and workflow explanation, not a substitute for the office’s procedure or professional advice. The examples on this site are fictional.