Add the option to load legacy content message.

load-legacy-pmpro-content-message.php

<?php
/**
 * Run this code _once_ and then remove it from your site. To run this code visit any page of the admin area.
 * After visiting any admin page, you may delete this script from your site.
 * This will add legacy logic to the content message for non-members.
 */
 function my_pmpro_legacy_content_message() {
   // Only logged-in admins can run this.
   if ( ! current_user_can( 'manage_options' ) || ! empty( get_option( 'pmpro_nonmembertext' ) ) ) {
       return;
   }

   $default_text = 'This content is for !!levels!! members only.<br /><a href="!!login_url!!">Log In</a> <a href="!!levels_page_url!!">Join Now</a>';
   update_option( 'pmpro_nonmembertext', $default_text );
 }
 add_action( 'admin_init', 'my_pmpro_legacy_content_message' );