The jury may still be out about whether or not Clark will manage to slip on his Super-suit before the end of Smallville's tenth and final season (He will, let's face it), but it's official that his cousin will look pretty familiar when she returns...
Executive producer Kelly Souders told TV Guide Magazine,
You really see her as Supergirl... Not the whole outfit, but her wardrobe will have evolved even a little more than Clark's. She's less Kara and way more Supergirl.
Oh, and she'll not only be Supergirl, but also Linda Lee Danvers, as well, according to actress Laura Vandervoort:
You'll see Kara in the [brunette] wig and glasses [as] a bit of an homage to her other identity from the comics, Linda Lee Danvers.
Somewhere, Supergirl comic writer Sterling Gates is in seventh heaven (Oh, there he is).