Add a "Grace Period" to a Paid Memberships Pro Membership (After expiration date is met).

pmpro-grace-period.php

<?php
/*
 * The Code Recipe will add a 15-day grace period to a members membership once the membership expires.
 * Add this Code Recipe to a PMPro Customization Plugin - https://www.paidmembershipspro.com/create-a-plugin-for-pmpro-customizations/
 * For more information on Paid Memberships Pro - www.paidmembershipspro.com
 */

function my_pmpro_membership_post_membership_expiry( $user_id, $level_id ) {
	// Make sure we aren't already in a grace period for this level
	$grace_level = get_user_meta( $user_id, 'grace_level', true );
	if ( empty( $grace_level ) || $grace_level !== $level_id ) {
		// Give them their level back with 15 day expiration
		$grace_level                  = array();
		$grace_level['user_id'] = $user_id;
		$grace_level['membership_id'] = $level_id;
		$grace_level['enddate']       = date( 'Y-m-d H:i:s', strtotime( '+15 days', current_time( 'timestamp' ) ) );
		$changed = pmpro_changeMembershipLevel( $grace_level, $user_id );
		update_user_meta( $user_id, 'grace_level', $level_id );
	}
	else
		delete_user_meta($user_id, 'grace_level');
}

add_action('pmpro_membership_post_membership_expiry', 'my_pmpro_membership_post_membership_expiry', 10, 2);

This recipe is included in the blog post on "Add a Grace Period to a Membership Level with PMPro" at Paid Memberships Pro here: https://www.paidmembershipspro.com/add-a-grace-period-to-a-pmpro-membership/