Set the initial amount to $0 for existing members.

set-initial-price-for-members.php

<?php
/**
 * Set the initial price to xxx for members only, for specific levels.
 *
 * 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 ) {
    // Define different prices for specific levels
    $level_prices = array(
        3 => 50,  // Level ID 3 -> $50 initial payment
        5 => 75,  // Level ID 5 -> $75 initial payment
        8 => 50  // Level ID 7 -> $100 initial payment
    );

    // Check if the level ID exists in the array and update the initial payment
    if ( array_key_exists( $level->id, $level_prices ) ) {
        $level->initial_payment = $level_prices[$level->id];
    }

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