Technically...
She's never really implied being 'naked' (her profile not displaying a visible semblance of covering over the skin or body suit that isn't flush with her skin, at best) until Halo 4.

Halo CE & Anniversary: not really detailed enough anyway
Halo 2 & Anniversary: visible collar, especially on the more recent rendered cutscenes
Halo 3: Same visual style as 1 & 2 implying no real change in attire
Halo 4: Umm... No- or open-collar, significantly increased visual definition of lady-parts, minus the little bits that would effectively rate it an R depiction :P
(whatever the 'reason' for her new appearance, she's never implied being truly 'naked' in the past, as tight as her chosen attire/bodysuit was, but it's much easier to make the case for that aspect of her evolution since her appearance in Halo 4)

But I digress :P