How long ago did they book?
How likely are you to get another booking.
If you think you will struggle to rebook that week, I would advise that you won't be able to accept an alteration - but if the guest wants to cancel, then if you are able to rebook some or all of the time you would be happy to consider a refund for the 50% that would be witheld under the strict cancellation policy that they booked under.
In that way you are not out of pocket for a guests change of arrangement.