To replace checked/unchecked values in user emails to yes/no

gistfile1.php

<?php
 
function nf_change_user_email_value( $email ) {
  $original = array('checked','unchecked')
  $replace = array('yes','no')
  return str_replace( $original, $replace, $email );
}
add_filter( 'ninja_forms_email_user_value', 'nf_change_user_email_value' );

It looks like it might work, but you'll need to assign the str_replace() to $email before you return. Otherwise, you won't actually be modifying anything.