I tried searching for the answer but could only find how to change the default which I have already done.

Is it possible to force Zen Cart to always send emails in html format regardless of setting chosen during registration?

I've already changed the default during registration to html (setting 1), but I want to force html for those customers that registered a long time ago when I had the default as text.

If you know how to do this, please let me know.

I'm looking at includes/functions/function_email.php but I really am not sure what to change there. If this is even the right file.

Thanks!