Simple Proration for Upgrades Example

my_pmpro_checkout_level.php

<?php

/**
 * Simple Proration for Upgrades Example
 */
function my_pmpro_checkout_level( $checkout_level ) {
    
    // Get user's current level.
    $user_level = pmpro_getMembershipLevelForUser();
    
    // Bail if there is no level or checking out for the same level.
    if ( empty ( $user_level ) || $user_level->id == $checkout_level->id ) {
        return $checkout_level;
    }
    
    // Calculate prorated cost based on current level settings.
    $checkout_level->initial_payment = min( $checkout_level->initial_payment - $user_level->initial_payment, 0 );
    
    return $checkout_level;
}
add_filter( 'pmpro_checkout_level', 'my_pmpro_checkout_level' );

Line 17 needs to be changed from "min" to "max"