Larian has not explained exactly how the kickstarter alpha/beta keys will be revoked and re-activation done (if you choose to get the Steam version). Presumable the last kickstarter update will provide instructions.

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.