You can absolutely switch to the annual subscription! All you need to do is login to your account and on your ACCOUNT DASHBOARD click the CHANGE SUBSCRIPTION option. Any time left you have on your current monthly subscription pass will be added on to your annual subscription. Please note the offer is currently not available for NZ residents.
If your annual subscription is ending and you would like to continue for another year you'll need to cancel your current annual subscription and then re-subscribe to the annual offer for another year.