The T&Cs changed on 2nd May 2018 to specifically say tickets needed to be for the correct direction of travel(with an exemption for tickets bought before then)
see ‘can I use my ticket in either direction’ on the link below.
The FAQs also say of yiu have bought the wrong ticket to contact customer services (9am - 5pm Monday to Friday !)
https://www.heathrowexpress.com/faqs