Larian already stated that you'll be able to change to the GOG version later on. Your Steam key will then simply stop working and the game will be removed from your Steam account.
The full game codes are the same as the Early Access codes, and "updating" the game to the latest patch will not invalidate your ability to change to a GOG key later.
Keep in mind that if you decide to change to the GOG key, you won't be able to change back to the Steam version again. This choice will be final.
