@Carmen802
With a long term booking ( 27 days plus) a guest can make changes to their stay.
It can be done automatically without host approval, if they meet the conditions - that is after the stay starts, they still have 30 days or more before the checkout date as the cancellation policy gives the host 30 days paid notice.
When a trip has started, but it’s less than 30 days before the checkout date, the host needs to approve this change - so here you can compromise to make sure you receive the full 30 days notice.
Changes to trips to below 27 days would mean the long term discount applied to the booking would be lost to the guest, and the trip cost would be recalculated, so your thought that the change was done to get extra discount isn't right.
What I'd say, if you allow airbnb bookings over 30 days, you should keep in mind your guest can make auto adjustments not always in your favour and without penalty. Either you can decide to accept this or you can cap your maximum booking length to 28-30 days to deter auto adjustments.
By doing this a guest could still book for two months potentially, but then they would have to make separate bookings, but this gives your better protection against amendments.