Customize automatic email only once for every client

[blueberry]

Hi,

I need to send modified email "Invoice Created" with merge fields but only once for every client. After that one time, I want default "Invoice Created" template to be sent. 
As these emails are send automatically, how can I execute my hook EmailPreSend (add merge fields to template) only for first client's invoice created after appyling my changes? 
I need solution that would check if merge fields were sent in previous Invoice Created email, if not, they will be added to template.

Or maybe I should create custom template and then find out how to send it instead of default "Invoice Created" template for that one time?

Please help