my_pmpro_stop_level_buying_anotherlevel.php

<?php

/**
 * This recipe will restrict someone with level 1 changing to level 2.
 *  change line 25 for other level numbers
 * You can add this recipe to your site by creating a custom plugin
 * or using the Code Snippets plugin available for free in the WordPress repository.
 * Read this companion article for step-by-step directions on either method.
 * https://www.paidmembershipspro.com/create-a-plugin-for-pmpro-customizations/
 */

function my_pmpro_restrict_membership_by_level( $okay ) {
 		
 		// If something else isn't okay, stop from running this code further.
 		if ( ! $okay ) {
 			return $okay;
 		}

 		// If the user doesn't have a membership level carry on with checkout.
 		if ( ! pmpro_hasMembershipLevel() ) {
 			return $okay;
 		}

 		// Check if the user's current membership level is restricted from chaning to new level.
 		if ( pmpro_hasMembershipLevel( '1' ) && $_REQUEST['level'] == '2' ) { // Change level ID to a different level.
 			$okay = false;
 			pmpro_setMessage( 'This Level is for new Members Only', 'pmpro_error' );
 		}
 		return $okay;

} 
add_filter( 'pmpro_registration_checks', 'my_pmpro_restrict_membership_by_level', 10, 1 );