Jenny, I'm not sure it was the same performance.   In fact, now that I think about it, it probably wasn't, because I think I heard a tape of the whole performance with the "wish you had hair" ad lib followed by 2 minutes of Elvis laughing against the band's backup, and I don't think there was another gag in it.

It was the same song, though, and maybe once Elvis had originally 'deflowered' that song, he felt inclined to pull off similar shenanigans when singing it on other occasions.

Elvis was pretty far gone with the prescription meds by that time and his Las Vegas act was suffering for that. He died (allegedly) ;) a couple months later, I think.