If the guest cannot attend then their options are to request an alteration or a cancellation. If there is extenuation circumstances then the guest will have the opportunity to demonstrate this to Airbnb you won't need to concern yourself they will sort any proof needed. If you get a formal refund request you can deal with it then, full refund partial or not, you can choose.
Keeping the existing reservation and moving the dates will avoid the guest having a financial penalty and allow them to have another attempt at staying. But there is a down side for you as a host.
As a host you will not be reimbursed for the dates discarded, unless you can rebook them.
Your cancellation policy will now apply to the new dates and if the guest cancels them early enough they may be able to facilitate a full refund depending on the terms in your policy. For whatever period these new dates appear as booked or blocked on your calendare. You cannot book them. Your payment will now be postposed untill the new dates as well.
terms in your policy. For whatever period thes new dates appear blocked on your calendar