Any changes in the rates made by the host or ABB since you made your booking will affect the new pricing so you could just talk to the host, explain the situation and arrange to keep the reservation as is for now but ask the host if they'd be willing to refund a certain amount for the change in no. of guests after you check in and the host is able to confirm the total no. of guests that actually stayed.
I had a similar situation with my guest when she tried to change her check in date to 1 day later (about a month after her making the reservation and more than 3 months remaining till the check in date) due to a change in her flight schedule she got a higher total so I just told my guest let's agree to keep it as is, and I will refund you X amount (which was based on my payout amount rather than what the guest paid) for that 1 night once you check in. I was planning to block the day before anyway because I didn't want to do a same day turnover so it worked out for us.