Ten Ton Hammer has conjured up a short interview with City of Heroes lead producer Matt Miller about the game's upcoming Going Rogue expansion pack.
(Villains don't become vigilantes, they become rogues. They become the redemptive sort-of guys that think, '˜Maybe I'm fighting on the wrong side. Maybe I'm not doing everything for the right reasons. Maybe there's a better way for me to use my powers.' They'll get what I term internally as the (Han Solo missions,) the heart-of-gold but wears a grey hat type stuff. Eventually if they do enough of that stuff, they'll be invited over to Paragon City to become a full fledged hero. )
Even if you meet the content requirements to change allegiances, there's no danger of accidentally changing sides. (You'll have to purposefully do the content, and purposefully make that transition. So you can kind of ride that middle line I talked about - you can really stay a gray hat, and there will be some advantages to doing so. You can go from one zone to another more freely, but you'll lose some opportunities as well.)