The easiest thing to do is to just keep the reservation as is and have the people who are actually eating there give the name of the current lead guest when they check in. It doesn't matter if that guest isn't going and there is no penalty for showing up with less people.
If there is an absolute necessity to change names, yes, you have to cancel and hope to get another reservation. Or simply look for a new reservation for the people now going and then cancel the old one.