While we all know what is right and what are cancellation policies say, but the reality is that when a guest sends a change request, there isn't much of a choice. We can try to reason with the guest but that's as far as it goes. Trust me, I know and learned the hard way..
I had a guest booked for 17 days. While here (and by all accounts he was very happy with everything), he learned he had a friend of a friend who was going to allow him to stay for much much cheaper. I got his change request 1 hour before my checkout time on day 5.
AIRBNB essentially forced me (after many, calls from them and my guest) to finally accept it. The alternative was that he would return (he checked out) stay and leave me a bad review. The choices were clear.
Wish it were different (and I know it should be) but there you have it.