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

/*
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. 
*/
function bulk_lock_all_members()
{
	global $wpdb;
	$users = $wpdb->get_results( "SELECT ID FROM $wpdb->users" );
	if( $users ) {
	  foreach ( $users as $user ) {
	    update_user_meta( $user->ID, 'pmprolml', 1 );
	  }
	}
}

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