Extend booking
Allow a direct booking extension when guests book from sites such as Booking.com/Airbnb.
We currently have to create a new booking if a guest would like to extend their stay which then creates a request for ID/deposit etc and then they will get various messages which aren't necessary. We need the ability to extend an existing booking directly

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Keith commented
what you describe is extending the stay directly. they make a new reservation as a direct booking.
I think what you want is:
if airbnb, then use airbnb alteration to extend stay.and if booking.com use booking extend stay.
my personal preferences is to pocket the platform fees when guests extend. The platform isn't doing work so why should they get a commission on a guest wanting to enjoy our hospitality more.
So I charge them the same rate they paid on the platform and share the spoils with my owners.