For anyone who stumbled upon this thread in seach of the answer: After having two separate co-hosting agreements that required modifications, I had no other option than to call Airbnb to ask them how, if at all possible, do you amend an agreement. As I suspected, there is no way to make any changes to an agreement (only an open proposal can be amended). I would think this is probably an oversight, considering everything else is amendable, such as reservations.
So, for now, if you need to make any changes to an agreement, the host will need to resend co-hosting request, co-host then sends proposal, and host has to agree.