@Lucinda24
There is certainly no obligation for you to accept the amendment and airbnb tells guests this from the help pages;
-If the host accepts your request, your reservation will be updated and you’ll be charged or refunded if necessary. If the request is declined or you don’t receive a response, your reservation will stay the same-
You can explain to your prospective guest that as the change indeed goes below your minimum booking that you can't accept it and leave the ball in their court whether they wish to cancel the booking in view of this or proceed.
Don't you cancel, can be penalties, contact airbnb if you feel it is unavoidable.