@Fiona691 Usually when a guest does this, they're just trying to evade your cancellation policy. They have no intention of actually staying in your property 15 months from now - and even if they did, no person sane enough to be worthy of the keys to your house would think the vacation conditions in Bali in late 2022 were predictable at this complicated moment in history. You can safely assume that this person is not acting in good faith.
If it's currently possible for the guest to travel to your destination, you can assure them that you will honor their current booking, but insist that if they want to change to another date they will have to cancel under the terms of your current cancellation policy and send a new request when they know when they can travel again. If they get snippy about that, consider their bluff called. No matter what you do, though, I can guarantee you that altering this booking would not result in the guests staying with you in November 2022.