my_pmpro_registration_checks.php

/*
	Don't let exisisting members checkout for free level.
*/
function my_pmpro_registration_checks($okay)
{
	//only check if things are okay so far		
	if($okay)
	{
		global $pmpro_level;
				
		if(pmpro_hasMembershipLevel() && $pmpro_level->id == 7)
		{
			pmpro_setMessage("Your existing level has access to everything the Free level does. If you'd still like to change your level, please <a href='/membership-cancel/'>cancel first</a>.", "pmpro_error");
						
			$okay = false;
		}
	}

	return $okay;
}
add_action("pmpro_registration_checks", "my_pmpro_registration_checks");