Songs can change keys. Songs change keys all the time. A cool one to do is Em to Bm, or Em to G major since you're in Em already. Experiment, there's plenty you can do with metal, it doesn't have to be all in one key. Either way is fine, but key changes can be more interesting. Look at Franz Ferdinand. I know he's not metal, but his transitions and key changes are some of the best I've ever heard.