Hi,
it's probably NOT the best way (I mean it's a bad way...).
But for a quick change you can goto the file
\user\editadvanced_form.php
in the validation function you can replace the line 256 or near... (public function validation($usernew, $files) )
Replace
$usernew->username = trim($usernew->username);
by
$usernew->username = trim($usernew->email);
hope it's work !
Dominique.