Contract expire e-mails -- how to link e-mail template


We have started using the out of the box contracts module to track our customer contracts and have automatic reminders when contracts are about the expire. 

Right now, it is sending a standard e-mail saying the contract has been re-assigned which is not clear to the user. 

So we created a new e-mail template and would like to link this to the expiry. 

Does anyone know how to do this?