In an "effort to provided continued support" to Dark Souls players, FromSoftware and Bandai Namco Games have worked to provide a way for players to migrate their Dark Souls: Prepare to Die games, saves, and achievements from the Games for Windows Live platform to Steam.
Beginning November, there will be a period that allows you to transfer your Games for Windows Live Save Data and Achievements over to your Steam account; but, it will require that you redeem your Games for Windows Live Token (found here) to get Dark Souls: Prepare to Die Edition on Steam at no additional cost.
Should you miss this migration period, you'll still be able to get Dark Souls: Prepare to Die Edition for free on Steam, but your game Save Data and Achievements may not transfer.
Specific migration start and end dates will be announced soon; however, the migration period will begin sometime in November.
