From what I remember, feel free to correct with exact information .. Orochimaru can't perform his body-change jutsu very often. Something like once every two years or whatever. But I think he is skilled enough at illusionary jutsus that he can change his appearance at will, for the durarion of whatever chakra is required to perform the illusion. Think about when he appeared as his old face in his battle with the 3rd. His old body was long gone, but he could easily project himself as that face. Also, he showed himself earlier, presumably in the same body of that girl, as the sand Kage.

This, to me, explains it, but I could be wrong .. it's been a while since I read the parts related to this.

EDIT / Off-Topic : Could you guys please get some original avatars!?! It's getting really distracting to see the same avatars argue with each other rofl.