@Mike-And-Jane0
As far as I know I don't think there is any difference if guest or host initiates the change reservation request.
The new dates would be the same as if they booked them originally so the cancellation policy would apply as if it were a new booking.
I was surprised guest can initiate a change reservation request after check in. I only had this happen a few weeks ago. Guest even messaged that they loved the place, then 10 minuets later, messaged that they wanted to cancel their last 3 days of a 5 day stay. I responded back that they would need to cancel and I would refund if I rebooked the nights. They had booked 8 months prior. Said they didn't want to rent a car after all, even though they already asked where the closest Enterprise was some 8 months prior. Then I got the reservation alteration request. I approved it because I knew I could rebook the nights even on one days notice, and don't even care if I don't rebook. I just took the high road and said I hope they enjoy the rest of their trip. Otherwise would have gotten 1 star review. They ended up renting a car anyways, because they didn't understand the area, and had negotiated a discounted rate at a new listing. (I later read the review).
Just pointing out that be prepared to either grant request or risk one star review.
Why on earth would I allow longer stays? I maintain 95% occupancy rates with 5 night max. It's easier to rebook a few days instead of a few weeks. (Airbnb is always pushing for allowing longer stays)
ETA: These were also the only guest to take a bag of coffee and box of tea. I leave one for them and everyone just uses it however much they need. But these decided to take that with them to the other Airbnb listing they booked after checking into mine. Once again I don't care, but just pointing out how some guest are. It's just an honor system and they didn't violate any policy.