This works but is awkward in my opinion.

an invoice saved posts to the accounting, if after saving you change it to pending, it does  un-post from the accounting.

So if you did change the way the invoice template looks and what it says it is, the customer would think it was an estimate (while you have a pending invoice), then change the template back to an invoice, remove the pending tag, and save and then it will post to the accounting again.