Edit all PMPro email properties at once via pmpro_email_filter.

pmpro_email_filter_example.php

/*
  Edit email templates.
  
  Other fields to change:
  * $email->email
  * $email->from
  * $email->fromname
  * $email->subject
  * $email->template
  * $email->body
  * $email->headers
*/
function my_pmpro_email_filter($email)
{	
	$replace_data_again = false;
	
	if($email->template == "checkout_free")
	{
		//update subject
		$email->subject = "Thank you for your application.";
		
		//update body !! update this to point to a real email template file
		$email->body = file_get_contents(dirname(__FILE__) . "/emails/checkout_applicant.html");
		
		$replace_data_again = true;		
	}
		
	if($replace_data_again)
	{
		//replace data
		if(is_string($email->data))
			$email->data = array("body"=>$email->data);			
		if(is_array($email->data))
		{
			foreach($email->data as $key => $value)
			{
				$email->body = str_replace("!!" . $key . "!!", $value, $email->body);
			}
		}	
	}
		
	return $email;
}
add_filter("pmpro_email_filter", "my_pmpro_email_filter");