Clearly, he was reprising his role (and I'm sure I wasn't the only one who was happy to see it. If you haven't checked them out, MovieFone has a whole gallery of shots from that night, including about 15 of Grieco arriving in a police car and on the street outside the venue. I personally recommend # 12, wherein we learn through visuals that Richard has spent most of the years since we saw him in the original 21 Jump Street at the gym.
On a side note, Grieco looks to be having fun with premieres whether or not he has an associated character. The picture at the left is from the premiere of The Raven. If you saw the film, the look will probably strike you as quite appropriate (even familiar)--and it wouldn't be the only time.
AceShowBiz has shots of Grieco from several different premieres, and more often than not he looks to be in the spirit of the film.