The lights go down and we hear the Avengers theme.
Iron Man's helmet appears, audience cheers. Sound clips from all the Marvel movies start playing. I am Iron Man, I'm always angry,etc. I don't remember all of them, but they build to the end of the Avengers where Sam Jackson says they'll be back when they are needed.
All the while the camera sweeps in close to the Iron Man helmet. It's getting crunched, dented, misshapen. Sparks fly, crunching noises are heard. When we get that final Avengers quote the camera pulls back and the Iron Man helmet has been reforged as Ultron's helmet, an angry red light glowing out of his mouth and eye holes. Cue my eardrums being punctured by screaming fangirls (and boys, I'l be fair).
There you go. One of the best Holy **** moments of Comic-Con 2013.