Invoices being saved to Notes

I understand that by default when a user emails a generated invoice it automatically gets saved into the Notes module.

This is great, but we revise the invoice numerous times for each event we do so we end up clogging up the notes section with so many invoices with the same number and title etc, each more slightly improved.

How would I go about making it so that whenever an invoice is both generated OR emailed it gets saved into the Notes module and each time after that overwrites the existing one so that the most recent one is saved?

Any ideas?