When a letter is approved for sending, the approval covers exactly what was in front of the approver: the wording, the attachments and the list of people it goes to.
Any of these sends the letter back to review, and any queued sending is cancelled.
Because the alternative is an approval that covers something the approver never saw. Somebody approves a letter to two people, a third is added, and the record says it was approved.
Get it right before requesting approval. If something has to change afterwards, change it and ask again. The second request takes the same time as the first and there is no way around it.