Paid Memberships Pro - Prevent users from downgrading levels.

gistfile1.txt

/**
 * Prevent users from downgrading levels.
 */
add_filter( 'pmpro_checkout_level', 'my_pmpro_checkout_level' );
function my_pmpro_checkout_level( $level ) {
     // Check to see if the current user is downgrading (Gold to Silver or Gold to Free)
     if( pmpro_hasMembershipLevel( 3 ) && ( $level->id == 2 || $level->id == 1 ) ) {
          // Re-direct them to page explaining why they cannot downgrade
          wp_redirect( home_url( '/gold-downgrade-message' ) );
          exit;
     }
     // Check to see if the current user is downgrading (Silver to Free)
     else if( pmpro_hasMembershipLevel( 2 ) && $level->id == 1 ) {
          // Re-direct them to page explaining why they cannot downgrade
          wp_redirect( home_url( '/silver-downgrade-message' ) );
          exit;
     }

     return $level;
}