Well, actually right now I am experiencing trying to add a person and I don't claim to have a lot of experience, but online you can only try to find a new reservation. Like I needed to go from a table for 2 to a table for 3(4) because I didn't count my infant when I made my original ones.
I looked online constantly for a table for 3 and tried searching for 4 since it would be the same size table and there was nothing. But I called the same day and the CM instead of looking for a new reservation just tried to modify the one I had…something I don't see as an option online. She was able to modify 2 of my reservations. She modified my Chef Mickey's breakfast on 11/23 and 1900 Park Fare dinner on 11/25. My confirmation numbers stayed the same for both.
The other thing about calling is I can ask them to check every meal over a span of days, something else I don't see online except for CRT. It only lets you check one day and one meal at a time.
Of course the only time I am able to get into the online system is in the evenings whereas I can call from my office, wear my headset and work at the same time. Just saying, since you're trying to change a reservation as opposed to making a whole new one, it might be worth a phone call or 2.
I did get online at 5am this morning and found a new Crystal Palace reservation for us though.