my_pmpro_automatic_discount_application.php

<?php

// Copy from below here

/**
 * Automatically applies discount code for users switching from a level in a set of given levels.
 * Useful for giving discounts to members upgrading their memberships.
 * To use, make sure to update the first 2 variables in each function.
 */
function my_pmpro_automatic_discount_application() {
	$extended_expiration_discount_code = 'discountcode'; // Change to your discount code.
	$levels_users_can_switch_from = array( '1', '2' ); // List your level id's that users can switch from.

	if ( empty( $_REQUEST['level'] ) || ! empty( $_REQUEST['discount_code'] ) || ! pmpro_checkDiscountCode( $extended_expiration_discount_code, $_REQUEST['level'] ) ) {
		return;
	}

	$current_level = pmpro_getMembershipLevelForUser();
	if ( isset( $current_level->ID ) && in_array( $current_level->ID, $levels_users_can_switch_from ) ) {
		$_REQUEST['discount_code'] = $extended_expiration_discount_code;
	}
}
add_action( 'init', 'my_pmpro_automatic_discount_application' );

function my_pmpro_check_automatic_disocunt_code( $okay, $dbcode, $level_id, $code ) {
	$extended_expiration_discount_code = 'discountcode'; // Change to your discount code.
	$levels_users_can_switch_from = array( '1', '2' ); // List your level id's that users can switch from.

	if ( $extended_expiration_discount_code === $code ) {
		$current_level = pmpro_getMembershipLevelForUser();
		return in_array( $current_level->ID, $levels_users_can_switch_from ) ? $okay : false;
	}
	return $okay;
}
add_filter( 'pmpro_check_discount_code', 'my_pmpro_check_automatic_disocunt_code', 10, 4 );