If i remember correctly, in Episode 1, you see Sidious as you see him in 4,5 and 6. But, you don't see him get all funky looking until #3. Does Palpatine REALLY become Sidious? Does Sidious REALLY become the Emporer? WTF?
You see holograms of Sidious in TPM if that's what you're talking about....but his face wasn't deformed...it was normal, just mostly hidden.
Palpatine didn't become Sidious....he already was Sidious through the entire prequels working his schemes behind closed doors. Everything he did as "Palpatine" was an act in order to get control over the Senate.
It isn't until 3 however, that he is officially dubbed "The Emperor." This happens during the scene in which he says he's going to transform the Republic into the first Galactic Empire.