Hi, if you just want to give a discount in this special case, then you can do that by offering a special price. I would not play around with the guest number as this can end up with insurance problems, as already mentioned.
My little questions is, if you have a fee for additonal persons above 2 people, and you now want to make it cheaper, why? maybe rethink the fees for additional people if you find that it is too much. I base the addiotnal person fee on the extra costs involved, bedding, towels, showers etc. and do not need to deal with offering discounts after booking.... just an idea, good luck.