Thanks to When Fangirls Attack, a site with daily links about womens' image in comics, I found this link
It lists the global evolution of Villainesses from the 30's to the 21th century. Dangerous but Beautiful, to Reluctant Heroine, while passing through the Corrupted Heroine stage.
It's fairly complete.
I just would like to add a small detail: Heroines, it seems, are always beautiful, while villainesses go from Horrible harpies to Beautiful Babes.
But when a villainess goes to the other side, the package seems to include a complete makeover surgery. The two examples that spring to my mind are Rogue and Mystique.
I do not have copies of their first appearances here. i'll try to update that I could. But they were both ugly. I remember that episode of the X men where they go to erase the military files on them and wound up meeting Rogue...
But then, Rogue went to Charles Xavier, fearing for her psychic equilibrium. And lo and behold, she was suddenly cute as a fox.
Mystique never reaaaally became a good girl (but boy was she hot with those guns in X Factor!), she faked it. But still, she became quite the beauteous one (in a dangerous way... There's something about blue skin... Also, is it just me, or is Mother, from Cyberforce, identical? A question for another day, I suppose)
Out of the blue, these are the two that I remember. But I'm sure there are others!