I've never, not once, had a problem changing my party size when walking up to check in for a reservation, even during the most crowded times, either at DLR or WDW.
-Last Friday we checked into our CCR lunch, and they weren't even letting people into the building because they were "fully committed" for the day. The dining room was almost half empty.
-We had a dinner reservation for two at Be Our Guest during prime dinner hour, which is notoriously difficult to reserve. We showed up with 6, and were seated within ten minutes. In front of the fireplace, no less.
In both cases, calling to change the number of people would get the answer from a CM animatron that "nothing is available." Don't believe them. They only know how to read off a screen. The host at the restaurant will accommodate you.
Re: the "fire code" nonsense: any given restaurant has far fewer seats than fire code permits. Do you really think that people hired to read off a screen are going to be trusted to learn and follow a fire code?!