Confused about Heroes Season 3 (contains SPOLIERS, please do not read if you have not finished watching it)?

OK so Sylar becomes a shape shifter.....but didn't he already have that ability? Wasn't it in Season 1 that Sylar morphed into Nathan Petrelli and was shown giving a Presidential Speech? This does happen in Season 3 but I am confused....didn't it also happen in Season 1? And didn't Sylar thus already have the ability to shape shift? Which does not explain why he had to kill some one in Season 3 to get that ability..... Please explain, will be grateful for a clear answer!! Thanks.