Stop users from changing membership level on checkout on WooCommerce

dont-apply-level-for-members-woo.php

<?php
/**
 * Stop existing members from changing levels when purchasing a product on WooCommerce.
 * Add this code to your PMPro Customizations Plugin - https://www.paidmembershipspro.com/create-a-plugin-for-pmpro-customizations/
 */
 
 function woo_dont_apply_level_for_existing_members() {	
	
	if ( pmpro_hasMembershipLevel( '1' ) ) {
		remove_action( 'woocommerce_order_status_completed', 'pmprowoo_add_membership_from_order' );

	}

}
add_action( 'woocommerce_order_status_completed', 'woo_dont_apply_level_for_existing_members', 5 );