Why not call instead of doing it on line? I just called yesterday to change 2 dining ressies from 3 ppl to 4 (1900 Park Fare and an 815a Crystal Palace). Knowing the setup of tables at both of these restaurants, I thought it might be relatively easy to go from 3 to 4, and sure enough the reservation phone agent was able to up the # with no problem, even though the on-line system showed no availability.
Of course, going from 4 to 5 would probably be harder, or adding another person at a restaurant like Teppan Edo or the Biergarten (where you sit with other parties anyway)...but at most other restaurants a table for 3 is usually going to have 4 seats anyway
Good luck