But no matter which you buy, you get the same disc. You can install it over an existing Mac, or even over a Windows. You can install it once, or one hundred times. It doesn't matter.
Apple playing the software game in a modern way.
Apple has do away with license keys or other worthless preventive measures. It is now based on only EULA (license agreement) and your integrity.
Apple new policy: Reward honest people with a hassle-free product rather than trying to punish dishonest people by treating everyone like a criminal. Nice policy, I reckon.