@Sanna1
It's possible to make amendments even when the trip has started and is actually in progress.
Bear in mind it is a tool a savvy guest can use to circumvent your cancellation policy or just from a dizzy guest who doesn't understand operations from your point of view.
If you have a minimum stay, note it in your listings and if a guest attempts to make amendments below the minimum stay, you can reject it for this reason and give the guest a choice to keep the original or to cancel subject to the strict cancellation policy, especially if it is near the commencement of the trip.
You can be accommodating but you should stick to your rules too.