using user_save to change a role?

Hi everyone, I'm currently using the following code to assign roles to "paid" users in a function. The users are automatically assigned "unpaid" role, rid=7, on registration. I want to change it to "paid", rid=6, in this function.

The following bit of code successfully does this.

the variable $uid is available

	$result = db_query("DELETE FROM {users_roles} WHERE uid=%d AND rid=7", $uid, $rid);
      if ($result)
      {
      	watchdog('fee', "Removed 'Unpaid' role from user $name (User $uid, $mail)");
      }
      else
      {
      	watchdog('fee', "Failed to remove 'Unpaid' role from $name (User $uid, $mail)");
      }
	
	$result = db_query("INSERT INTO {users_roles} (uid, rid) values (%d, 6)", $uid, $rid);
	cache_clear_all($uid .':', 'cache_menu', TRUE);
      if ($result)
      {
		drupal_set_message("You have been successfully marked as paid.");
      	watchdog('fee', "Added 'Paid' role to user $name (User $uid, $mail)");
      }
      else
      {
	      drupal_set_message("We have failed to verify your payment. Please email us.");
      	watchdog('fee', "Failed to add 'Paid' role to $name (User $uid, $mail)");
      }

However, I want to use the function user_save to do this to maintain compatibility across other modules.
Please can someone help me