Show levels and allow checkout (renewal) for hidden levels if the user already has that level.

pmpro-allow-hidden-level-checkout.php

/*
Show levels which are hidden on the level page if the user already has that level.
*/
function my_pmpro_levels_array($levels)
{
	$member_level = pmpro_getMembershipLevelForUser($current_user->ID);
	
	$check = false;
	foreach($levels as $level)
	{
		if($level->id == $member_level->ID)
		{
			$check = true;
			break;
		}
	}
	
	if($check || !$member_level)
	{
		$newlevels=$levels;
	} 
	else
	{
		$newlevels = array_merge($levels, array($member_level)); 
	}
	
	return $newlevels;
}
add_filter("pmpro_levels_array", "my_pmpro_levels_array");

function my_pmpro_checkout_level($pmpro_level)
{
	global $wpdb;
	$pmpro_level = $wpdb->get_row("SELECT * FROM $wpdb->pmpro_membership_levels WHERE id = '" . esc_sql($_REQUEST['level']) . "' LIMIT 1");
	return $pmpro_level;
}

add_filter("pmpro_checkout_level", "my_pmpro_checkout_level");