It is unfortunately not possible to change the display name format in an email message, as this is hard coded into the email header which is added after the email has been Sent Final from NetCommunity or Online Express.

The format in which it displays is <First name> <First initial of Middle name> <Surname> - this information is pulled directly from the constituent record in RE if the email is being sent to a constituent list.
The only way to change the format of the name in the email would be to update the constituent record to reflect which name format the constituent wishes to be addressed by.