In Hueco Mundo, Aizen is first seen wearing Arrancar clothes over his Shihakusho. Afterwards, but before his transformation, Aizen wore full clothing similar in style to the Arrancar.
Prior to his departure from the Soul Society, he appeared as a mild-featured man with brown scholarly hair enhanced by square glasses. He wore the standard Shinigami uniform with a captain's haori. However, during his ascent toHueco Mundo, he removed his glasses and swept his hand through his hair, revealing menacing eyes and a very different hairstyle (swept back hair, with a strand hanging in his face). As he had previously revealed to Renji Abarai, the Aizen they all knew never existed and this would seem to be a visual representation of that claim.