You shouldn't have to do all of that. I've beta tested other games on Steam, and when the beta expired, it removed the game from the Steam list and my account, but it doesn't delete the files. Plugging in the release key should just patch the existing files.
Thanks for the clarification. Though I've used Steam for over 10 years (well before the HL2 launch), this is the first instance I've encountered of a beta expiring and needing a brand new key for the final product.
*EDIT*
In retrospect, Dragon Commander did, too, but it was handled differently from D: OS, since the beta never disappeared from our Steam inventory.
