On the website, making a change of number of guests is basically rebooking with current rate. Called customer service and it's the same on their side.

I always make reservation for "1 adult" since it is the default option and I am too lazy to update that. This is not an issue for me until this time, and unfortunately it's a Disney world resort, and it says the number of adults and children must be accurate in order to apply park related benefits to all guests.