Maintain Initial Payment amount from original checkout when members renew Level 1.

my_pmpro_maintain_initial_payment_amount_on_renewal.php

<?php
/**
 * Maintain Initial Payment amount from original checkout when members renew
 * level 1. Keeps existing active members on old price after changing level cost. 
 *
 * You can add this recipe to your site by creating a custom plugin
 * or using the Code Snippets plugin available for free in the WordPress repository.
 * Read this companion article for step-by-step directions on either method.
 * https://www.paidmembershipspro.com/create-a-plugin-for-pmpro-customizations/
 */
function my_pmpro_maintain_initial_payment_amount_on_renewal( $level ) {
	global $current_user;

	// Bail if user isn't logged in.
	if ( empty( $current_user->ID ) ) {
		return $level;
	}

	// Bail if not a level 1 checkout.
	if ( 1 !== (int) $level->id ) {
		return $level;
	}

	// Bail if user does not have level 1.
	if ( ! pmpro_hasMembershipLevel( 1 ) ) {
		return $level;
	}

	// Get amount paid by member for level 1.
	$user_level = pmpro_getSpecificMembershipLevelForUser( $current_user->ID, 1 );
	$user_paid  = (float) $user_level->initial_payment;

	// Get level 1 checkout price.
	$level_price = (float) $level->initial_payment;

	// Bail if checkout price and last payment amount are equal.
	if ( $level_price === $user_paid ) {
		return $level;
	}

	// Last initial payment amount is different from current level cost.
	// Set checkout price to last payment amount.
	$level->initial_payment = $user_paid;

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