That's not how it works. To change the party size they have to cancel the old reservation and book a new one, so there has to be a new reservation available for the larger party size. Even the phone agents can't just add a person to an existing reservation.
Don't cancel your old reservations unless you have a new one lined up already. You could always try and show up with an extra person. There is, however, a chance the group will be turned away with an extra person, especially at the ultrapopular restaurants. You should keep trying to find a new reservation for these with the larger party size, especially if you are going from an even number to an odd number.