Thanks @Gwen386 for being so prompt! (I was hanging on the phone, waiting for Airbnb help)
I checked, and instant book isn't on.
I don't know what a change request is (too late now for me, see below) but I will check this out.
I had to act quickly, so I declined her booking and then sent her a special offer, which then had to exclude the day she had requested. I've messaged her privately (she is a friend's colleague, visiting from overseas) explaining that I will definitely be welcoming her on the original day she requested and for the full period of the special offer.
A solution, of sorts, in the end.
Sometimes I find Airbnb's policies not helpful and maybe this is another occasion?