An example of how to use `pmpro_checkout_level` to redirect and interject communication with users during checkout. Sample taken from an attempt to prevent downgrading.

pmpro-redirect-during-checkout.php

<?php // Do not copy

/**
 * pmpro_checkout_level Use `pmpro_checkout_level` to redirect and interject communication with users during checkout.
 *
 * @param  array $levels The array created in admin
 *
 * @return array         Output of the array
 */
function pmpro_redirect_during_checkout( $level ) {

    $current_level = $level->id;
    if ( is_string( $current_level ) )
    {
        $current_level = intval($current_level);
    }

    // Check to see if the current user is downgrading (Gold to Silver or Gold to Free)
   if ( pmpro_hasMembershipLevel( 3 ) && ( 1 === $current_level || 2 === $current_level ) ) {
        // Re-direct them to levels page
        wp_redirect( home_url( '/membership-levels' ) );
        exit;
   } // Check to see if the current user is downgrading (Silver to Free)
   else if ( pmpro_hasMembershipLevel( 2 ) && 1 === $current_level ) {
        // Re-direct them to page explaining why they cannot downgrade
        wp_redirect( home_url( '/silver-downgrade-message' ) );
        exit;
   }
    return $level;
}
add_filter( 'pmpro_checkout_level', 'pmpro_redirect_during_checkout' );