Stop users from purchasing a lower membership level Paid Memberships Pro WooCommerce

make_lower_product_not_purchasable.php

<?php

/** 
 * This stops users from purchasing a lower level membership through WooCommerce. (Assume lower level ID is '1')
 * If user has level 2 or 3, stop them from purchasing level 1 membership.
 * Add this code to your PMPro Customizations Plugin - https://www.paidmembershipspro.com/create-a-plugin-for-pmpro-customizations/
 */
 
 // Stop users from purchasing a lower membership level via WooCommerce.
function pmpro_lower_level_is_purchasable( $is_purchasable, $product ) {

	if( pmpro_hasMembershipLevel( array( '2', '3' ) ) ) { //change '2','3' to higher levels to stop users from signing up for a 'lower' level.
		if( 80 == $product->id ) { //change product ID for lower level product ID.
			$is_purchasable = false;
		}
	}

	return $is_purchasable;

}
add_filter( 'woocommerce_is_purchasable', 'pmpro_lower_level_is_purchasable', 10, 2 );