// Hook Into Order Status Change
add_action(
	'woocommerce_order_status_changed',
	function( $order_id, $status_from, $status_to ) {

		// Only For Change To This Status
		if( $status_to != 'TARGET-ORDER-STATUS' ) { return; }

		// Get Woo Emails
		$mailer = WC()->mailer();
		$mails = $mailer->get_emails();
		if( empty( $mails ) ) { return; }
		foreach( $mails as $mail ) {

			// Get Completed Email
			if( $mail->id != 'customer_completed_order' ) {
				continue;
			}

			// Replace Subject Line
			add_filter(
				'woocommerce_email_subject_customer_completed_order',
				function( $subject ) {
					return 'NEW SUBJECT LINE GOES HERE';
				}
			);

			// Replace Heading
			remove_action( 'woocommerce_email_header', array( WC()->mailer(), 'email_header' ) );
			add_action( 'woocommerce_email_header', function( $email_heading, $email ) {
				if( $email->id == 'customer_completed_order' ) {
					$email_heading = 'NEW EMAIL HEADING GOES HERE';
				}
				wc_get_template(
					'emails/email-header.php',
					[ 'email_heading' => $email_heading ]
				);
			}, 10, 2 );

			// Replace Content
			add_filter( 'gettext', function( $original ) {
				switch( $text ) {
					case 'We have finished processing your order.':
						return 'NEW BODY CONTENT GOES HERE';
				}
				return $original;
			} );

			// Send It
			$mail->trigger( $order_id );

		} // End Emails Loop

	}, 10, 3
);