Prorate Paid Memberships Pro Checkouts based on initial payments only.

prorate-pmpro-based-on-initial-payment.php

<?php
/**
 * Prorate initial payments (only) for Paid Memberships Pro checkouts.
 * This will prorate for any active members changing levels. If the level is the same price, it does not prorate to $0.
 * If a member is 'downgrading' levels, set the price to $0. (i.e. $10 -> $5 level will set the initial payment to $0).
 * 
 * Add this code to your site by following this guide - https://www.paidmembershipspro.com/create-a-plugin-for-pmpro-customizations/
 */
 
 function pmpro_prorate_levels_price( $level ) {
	global $current_user;

	// If user is logged-out, just bail.
	if ( ! is_user_logged_in() ) {
		return $level;
	}

	// If user doesn't have an active level, don't prorate.
	if ( ! pmpro_hasMembershipLevel() ) {
		return $level;
	}
	$user_level = pmpro_getMembershipLevelForUser( $current_user->ID );
	
	$user_paid = (float) $user_level->initial_payment;
	$level_price = (float) $level->initial_payment;

	// If they are downgrading, set the price to $0 for now.
	if ( $user_paid > $level_price ) {
		$level->initial_payment = 0;
		return $level;
	}

	// If the price is exact same as the amount paid, just leave it. We can assume it's a renewal. Remove this code if you want to set it to $0.
	if ( $level_price === $user_paid ) {
		return $level;
	}
	
	// Adjust price (prorated now) to the difference.
	$level->initial_payment = $level_price - $user_paid;

	return $level;
}
add_filter( 'pmpro_checkout_level', 'pmpro_prorate_levels_price', 20, 1 );