Funcom is paving the way to a hopefully smooth release with an informative FAQ posted on the SWL site today. The studio notes that the Steam client is different from its own client but that both clients will access the same game servers.
The FAQ covers all of the possible scenarios, including whether players had owned the game in the past, had played on Steam, been playing for the past month, and so on. If you have been accessing Funcom’s SWL client but would rather change it to the Steam client for accessibility and consolidation’s sake, you can — but it is a one-way, one-time process if chosen.
Source: Secret World Legends