PMPro Lock Membership Levels-lock existing members. Add this to your customizations plugin and refresh the page. Recommend deleting the code snippet after the operation is complete.

pmpro-lock-membership-levels-lock-all-existing-members.php

<?php
/**
 * Bulk Update all member's with level ID 4 to be locked indefinitely.
 * Run this code by adding to your site, loading a page. Once done, you may remove this code from your site.
 */

function bulk_lock_all_members() {
	
	if ( ! current_user_can( 'manage_options' ) ) {
		return;
	}
	
	global $wpdb;
	$users = $wpdb->get_results( "SELECT user_id FROM $wpdb->pmpro_memberships_users WHERE membership_id = 4 AND status = 'active'

" );
	if( $users ) {
	  foreach ( $users as $user ) {
	    update_user_meta( $user->ID, 'pmprolml', 1 );
	  }
	}
}

add_action( "init", "bulk_lock_all_members", 10, 0 );