Redirect away from the checkout page if you don't have a required level with Paid Memberships Pro.

my_template_redirect_upgrade.php

/*
	Redirect away from the checkout page if you don't have a required level.
*/
function my_template_redirect_upgrade()
{
	global $pmpro_pages, $pmpro_level;
	
	if(empty($pmpro_pages))
		return;
		
	if(is_page($pmpro_pages['checkout']) && $pmpro_level->id == 2 && !pmpro_hasMembershipLevel(1))	//change ids checked here
	{
		wp_redirect(site_url('page-explaining-you-need-level-1-first'));		//change url here
		exit;
	}
}
add_action('template_redirect', 'my_template_redirect_upgrade');