Bandai Namco announced on its Tumblr that in December, Dark Souls: Prepare to Die will be dropping Games for Windows Live to transition to Steam. They outlined the changes and the process, saying that GFWL users will receive a token to redeem on Steam at no additional cost. The token is available through a support link.
This month, Bandai Namco will also allow GFWL users to transfer saves and achievements from their old game to their Steam accounts also. You will be able to redeem Dark Souls: Prepare to Die even after the migration period ends, but save data and achievements won’t be transferred.