I’ve added the following 2 email merge tags to a WordPress membership plugin, and everything works perfectly. I know that this is not a good idea as the changes will get over written when the plugin is updated. However, I’m struggling to workout how to add my changes without directly editing the plugin. I’ve tried a few things in the functions.php, but non of them worked.

Any help or guidance would be gratefully received.

I want to add the following:

'membership_30'             => date_i18n( wc_date_format(), $expiration_date_timestamp - 30 * DAY_IN_SECONDS ),
'membership_14'             => date_i18n( wc_date_format(), $expiration_date_timestamp - 14 * DAY_IN_SECONDS ),

to the following section:

$email_merge_tags = array(

of this file:

<?php
/**
 * WooCommerce Memberships
 *
 * This source file is subject to the GNU General Public License v3.0
 * that is bundled with this package in the file license.txt.
 * It is also available through the world-wide-web at this URL:
 * http://www.gnu.org/licenses/gpl-3.0.html
 * If you did not receive a copy of the license and are unable to
 * obtain it through the world-wide-web, please send an email
 * to license@skyverge.com so we can send you a copy immediately.
 *
 * DISCLAIMER
 *
 * Do not edit or add to this file if you wish to upgrade WooCommerce Memberships to newer
 * versions in the future. If you wish to customize WooCommerce Memberships for your
 * needs please refer to https://docs.woocommerce.com/document/woocommerce-memberships/ for more information.
 *
 * @author    SkyVerge
 * @copyright Copyright (c) 2014-2020, SkyVerge, Inc. (info@skyverge.com)
 * @license   http://www.gnu.org/licenses/gpl-3.0.html GNU General Public License v3.0
 */

use SkyVergeWooCommercePluginFrameworkv5_5_0 as Framework;

defined( 'ABSPATH' ) or exit;

/**
 * User Membership notification emails abstract class.
 *
 * Handles common methods and hooks for emails  related to a user membership's events.
 *
 * @since 1.7.0
 */
abstract class WC_Memberships_User_Membership_Email extends WC_Email {


    /** @var string the email content body */
    protected $body = '';

    /** @var bool whether the email should be send to an admin (default false) */
    protected $sent_to_admin = false;

    /** @var int default schedule (optional, used by some emails) */
    protected $default_schedule = 0;

    /** @var string rescheduling events action description (optional, used by some emails) */
    protected $reschedule_description = '';

    /** @var bool whether the contents of the email can be edited from a Membership Plan */
    protected $plan_editable = false;


    /**
     * Membership email constructor.
     *
     * @since 1.14.0
     */
    public function __construct() {

        if ( $this->plan_editable ) {

            $this->description .= ' ' . sprintf(
                /* translators: Placeholders: %1$s - Opening <a> HTML tag, %2$s - Closing </a> HTML tag */
                __( 'You can edit the content of this email for %1$seach one of your plans%2$s individually.', 'woocommerce-memberships' ),
                '<a href="' . esc_url( admin_url( 'edit.php?post_type=wc_membership_plan' )  ) . '">', '</a>'
            );
        }

        parent::__construct();
    }


    /**
     * Checks if it's a customer email.
     *
     * @since 1.7.0
     *
     * @return true overrides parent method to always return true
     */
    public function is_customer_email() {
        return true;
    }


    /**
     * Parses the email's body merge tags.
     *
     * @since 1.7.0
     */
    protected function parse_merge_tags() {

        if ( ! $this->object instanceof WC_Memberships_User_Membership ) {
            return;
        }

        $user_membership = $this->object;
        $membership_plan = $user_membership->get_plan();

        // get member data
        $member            = get_user_by( 'id', $user_membership->get_user_id() );
        $member_name       = ! empty( $member->display_name ) ? $member->display_name : '';
        $member_first_name = ! empty( $member->first_name )   ? $member->first_name   : $member_name;
        $member_last_name  = ! empty( $member->last_name )    ? $member->last_name    : '';
        $member_full_name  = $member_first_name && $member_last_name ? $member_first_name . ' ' . $member->last_name : $member_name;

        // membership expiry date
        $expiration_date_timestamp = $user_membership->get_local_end_date( 'timestamp' );

        // placeholders
        $email_merge_tags = array(
            'member_name'                 => $member_name,
            'member_first_name'           => $member_first_name,
            'member_last_name'            => $member_last_name,
            'member_full_name'            => $member_full_name,
            'membership_plan'             => $membership_plan ? $membership_plan->get_name() : '',
            'membership_expiration_date'  => date_i18n( wc_date_format(), $expiration_date_timestamp ),
            'membership_expiry_time_diff' => human_time_diff( current_time( 'timestamp', true ), $expiration_date_timestamp ),
            'membership_view_url'         => esc_url( $user_membership->get_view_membership_url() ),
            'membership_renewal_url'      => esc_url( $user_membership->get_renew_membership_url() ),
        );

        if ( FrameworkSV_WC_Plugin_Compatibility::is_wc_version_gte( '3.2' ) ) {

            foreach ( $email_merge_tags as $find => $replace ) {
                $this->placeholders[ '{' . $find . '}' ] = $replace;
            }

        } else {

            foreach ( $email_merge_tags as $find => $replace ) {
                $this->find[ $find ]    = '{' . $find . '}';
                $this->replace[ $find ] = $replace;
            }
        }
    }


    /**
     * Returns the email default body content.
     *
     * This method should be overridden by child classes.
     *
     * @since 1.7.0
     *
     * @return string HTML
     */
    public function get_default_body() {
        return '';
    }


    /**
     * Returns the email body content.
     *
     * @since 1.7.0
     *
     * @return string HTML
     */
    public function get_body() {

        $email_id = strtolower( $this->id );

        /**
         * Filters the membership email body.
         *
         * @since 1.7.0
         *
         * @param string $body email body content
         * @param WC_Memberships_User_Membership_Email current email instance
         */
        $body = (string) apply_filters( "{$email_id}_email_body", $this->format_string( $this->body ), $this->object );

        if ( empty( $body ) || ! is_string( $body ) || '' === trim( $body ) ) {
            $body = $this->get_default_body();
        }

        // convert relative URLs to absolute for links href and images src attributes
        $domain  = get_home_url();
        $replace = array();
        $replace['/href="(?!https?://)(?!data:)(?!#)/'] = 'href="' . $domain;
        $replace['/src="(?!https?://)(?!data:)(?!#)/']  = 'src="'  . $domain;

        $body = preg_replace( array_keys( $replace ), array_values( $replace ), $body );

        return $body;
    }


    /**
     * Enables the email.
     *
     * @since 1.11.0
     */
    public function enable() {

        $email_settings = get_option( "woocommerce_{$this->id}_settings", array() );

        $email_settings['enabled'] = 'yes';

        update_option( "woocommerce_{$this->id}_settings", $email_settings );

        $this->enabled = true;
    }


    /**
     * Disables the email.
     *
     * @since 1.11.0
     */
    public function disable() {

        $email_settings = get_option( "woocommerce_{$this->id}_settings", array() );

        $email_settings['enabled'] = 'no';

        update_option( "woocommerce_{$this->id}_settings", $email_settings );

        $this->enabled = false;
    }


    /**
     * Returns the email schedule, in days (if applicable).
     *
     * @since 1.11.0
     *
     * @return int number of days (minimum 1)
     */
    public function get_schedule() {

        $schedule = null;

        if ( ! empty( $this->schedule_name ) && is_string( $this->schedule_name ) ) {

            $email_settings = get_option( "woocommerce_{$this->id}_settings", array() );
            $schedule       = ! isset( $email_settings[ $this->schedule_name ] ) || ! is_numeric( $email_settings[ $this->schedule_name ] ) ? $this->default_schedule : $email_settings[ $this->schedule_name ];
        }

        return max( 1, (int) $schedule );
    }


    /**
     * Sets the email schedule (if applicable).
     *
     * @since 1.11.0
     *
     * @param null|int $days number of days to set the schedule (cannot be lower than 1)
     * @return false|int returns false upon failure or gives the number of days set upon success
     */
    public function set_schedule( $days = null ) {

        $success  = false;
        $schedule = null === $days ? $this->default_schedule : $days;

        if ( ! empty( $this->schedule_name ) && is_string( $this->schedule_name ) && is_numeric( $schedule ) ) {

            $schedule       = max( 1, absint( (int) $schedule ) );
            $email_settings = get_option( "woocommerce_{$this->id}_settings", array() );

            $email_settings[ $this->schedule_name ] = $schedule;

            if ( update_option( "woocommerce_{$this->id}_settings", $email_settings ) ) {
                $success = $schedule;
            }
        }

        return $success;
    }


    /**
     * Returns the arguments that should be passed to an email template.
     *
     * @since 1.12.0
     *
     * @param array $args default args
     * @return array associative array
     */
    protected function get_template_args( $args = array() ) {

        return array(
            'user_membership' => $this->object,
            'email'           => $this,
            'email_heading'   => $this->get_heading(),
            'email_body'      => $this->get_body(),
            'sent_to_admin'   => $this->sent_to_admin,
        );
    }


    /**
     * Returns the email HTML content.
     *
     * @since 1.7.0
     *
     * @return string HTML
     */
    public function get_content_html() {

        $args = array( 'plain_text' => false );

        ob_start();

        wc_get_template( $this->template_html, array_merge( $args, $this->get_template_args( $args ) ) );

        return ob_get_clean();
    }


    /**
     * Returns the email plain text content.
     *
     * @since 1.7.0
     *
     * @return string plain text
     */
    public function get_content_plain() {

        $args = array( 'plain_text' => true );

        ob_start();

        wc_get_template( $this->template_html, array_merge( $args, $this->get_template_args( $args ) ) );

        return ob_get_clean();
    }


}