I don't have a Lion license and I'm currently running Mac OS X 10.6.6 (Snow Leopard).
If I buy Mountain Lion, can I downgrade to Lion?
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If you buy Mountain Lion, your computer downloads an installer for Mountain Lion from the app store. If you were somehow able to buy a copy of Lion later on (which Apple does not seem to be selling in the App store), you could install it on your computer ("downgrade to Lion"), but buying Mountain Lion does not get you access to such an installer.
No OS X Lion license if you don't already have a OS X Lion Purchased in the Mac App Store prior to the release of Mountain Lion.
That means that you can't buy OS X Lion now from the Mac App Store even if you really wanted to do so. So when you buy Mountain Lion, all you obtain is the Mountain Lion Installer and license.
If you must have Lion your only legal easy recourse is to try and find one of the official Apple Lion USB install media.
Another probably not so easy option is to contact Apple and see if they can work some magic to allow you to purchase Lion, but as far as it goes you would have to purchase Lion, your not going to be granted access to a prior releases based on purchasing the current release of OS X.