Demandred never turned evilmhe just went over to the Shadow to enact revenge on LTT. So,if he succedded,i thnik he would switch sides again and become the new Dragon. Also he said he would kill anybody who kills Rand,so I think if DO succedeed in killing Rand in the first 4-5 books,he would switch over to the Light once more to enact revenge on the DO. It is also stated that the difference betwwen the two is almost inexisting,so Demandred would be a fine replacement.