There should hopefully just be a seamless update if you activate the new key. If not, you should be able to avoid re-downloading most of the game by copying or renaming the D:OS install folder before release, and pausing the download after release, moving the files to the new install folder and verifying local files.
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.
If you're concerned about re-downloading, use Steam's built in backup to backup D:OS. Then after your alpha key expires, when you get your release key, activate it on Steam but don't install it. Use the Steam backup to restore the D:OS backup and it will install from the backup and apply any updates during the restore.
