Keep pre-existing members on their initial checkout price when renewing their membership level.

keep-pmpro-member-price-manual-renewal.php

<?php
/**
 * Keep pre-existing members on their initial checkout price when manually renewing. Grandfather 'old' members when their levels pricing changes.
 * Note: This is not needed for recurring membership levels and is designed for levels with an expiration date.
 * 
 * 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_original_price_existing_members( $level ) {
	global $current_user;

	if ( ! is_user_logged_in() ) {
		return $level;
	}

	// If they don't have a level already, just keep the price normal.
	if ( ! pmpro_hasMembershipLevel() ) {
		return $level;
	}
	
	$user_level = pmpro_getMembershipLevelForUser( $current_user->ID );
	if ( $level->id === $user_level->id ) {
		// Let's adjust pricing here.
		$last_order = new MemberOrder();
		$last_order->getLastMemberOrder( $current_user->ID, array('success','cancelled'), $user_level->id );
  
   		// Only apply this to paid levels.
		if ( $level->initial_payment > 0 ) {
			$level->initial_payment = $last_order->subtotal;
		}

	}

	// Set price billing to original price paid way back when.
	return $level;
}
add_filter( 'pmpro_checkout_level', 'my_pmpro_original_price_existing_members', 10, 1 );