Set the initial amount to $0 for existing members.

set-initial-price-to-0-for-members.php

<?php
/**
 * Set the initial price to $0 for members only, for any level.
 *
 * Add this code to your site by following this guide - https://www.paidmembershipspro.com/create-a-plugin-for-pmpro-customizations/
 */

function my_pmpro_adjust_initial_amount( $level ) {

    if ( pmpro_hasMembershipLevel() ) {
        $level->initial_payment = 0;
    }

    return $level;
}
add_filter( 'pmpro_checkout_level', 'my_pmpro_adjust_initial_amount' );