As I understand the policy for long term renters (which are defined by AirBnB to be a rental of 28 days or more), that you can make changes to your reservation free of charge up to 30 days prior to your stay. I don't know for sure, since I am a host, but I believe they take at least the AirBnB fee + rent for 28 days of the stay (again would need to be confirmed by a guest that is a long term renter). Per their long term cancellation policy, they will adjust the AirBnB fee based on the change you make.
If you cancel, you will not get their fee back and I believe you are in for at least the first 30 days of your stay in terms of payment.
If you change within the 30 days of your stay, you need to go to the resolution center and work with the host to negotiate a different arrangement.
You can find the Long Term Cancellation policy on the website.