@Monica1228
1. A listing can have "Smart Pricing" activated on it, which means prices change dynamically without intervention of the host. So prices can be different then at time of booking.
2. It is well known the "change feature" not allways calculates correct prices. I would suggest to ask the host to change the reservation, as the host has the option to correct the calculated price manually.
3. Just for the changing of a reservation there are no costs involved (like airlines or agencies do). Please note when changing a reservation the actual cancellation policy replaces the old one (if host changed this setting recently)
Hope this help,
Emiel