Charge the level's Billing Amount instead of the Initial Payment when an existing member checks out to manually renew their current level.

charge-billing-amount-on-renewal.php

<?php
/**
 * Charge the level's Billing Amount instead of the Initial Payment when an
 * existing member checks out to manually renew their current level.
 *
 * Only applies to members who have been continuously active on that level
 * for at least the number of months set in MY_PMPRO_RENEWAL_ACTIVE_MONTHS below.
 *
 * 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/
 */

// How many months a member must have been active to get the renewal price.
define( 'MY_PMPRO_RENEWAL_ACTIVE_MONTHS', 6 );

function my_pmpro_renewal_billing_amount_at_checkout( $level ) {
	// Bail if there's no level or the visitor isn't logged in.
	if ( empty( $level ) || ! is_user_logged_in() ) {
		return $level;
	}

	// Only adjust pricing when the member is renewing the level they already have.
	if ( ! pmpro_hasMembershipLevel( $level->id ) ) {
		return $level;
	}

	// Bail if the level has no recurring Billing Amount to fall back on.
	if ( empty( $level->billing_amount ) ) {
		return $level;
	}

	// Get the start date (timestamp) of the member's current active membership for this level.
	$startdate = pmpro_getMemberStartdate( get_current_user_id(), $level->id );
	if ( empty( $startdate ) ) {
		return $level;
	}

	// Bail if they haven't been active long enough yet.
	$cutoff = strtotime( '-' . absint( MY_PMPRO_RENEWAL_ACTIVE_MONTHS ) . ' months', current_time( 'timestamp' ) );
	if ( $startdate > $cutoff ) {
		return $level;
	}

	// Qualified renewal: charge the Billing Amount today instead of the Initial Payment.
	$level->initial_payment = $level->billing_amount;

	return $level;
}
// Priority 5 so this runs before the Auto-Renewal Checkbox Add On (priority 7),
// which zeroes out billing_amount when a member opts out of auto-renewal.
add_filter( 'pmpro_checkout_level', 'my_pmpro_renewal_billing_amount_at_checkout', 5 );