I have a completely different opinion on the matter.
It's good that your guests have found permanent accommodation earlier than expected but you shouldn't be out financially because of it.
To be flexible, I would suggest to them that if they cancel the final night and you can find someone to replace them, you would be happy to consider a partial refund.
This doesn't mean you don't care about your guests or that you don't go the extra mile, simply that you are running a business and shouldn't be out financially because of a change in your guests plans.