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As I explained in Installing Lion: What you need to know, one of the requirements for installing Lion is that you already have Snow Leopard (Mac OS X 10.6) version 10.6.6 or later installed. The main practical reason for this requirement is that Lion is available only via the Mac App Store, and the Mac App Store debuted in Mac OS X 10.6.6. In other words, you need Snow Leopard just to purchase and download Lion.
But once you’ve got your copy of Lion, can you install it onto a Mac or a hard drive containing Leopard (Mac OS X 10.5)?
The software license you agree to when you install Lion states that you can “download, install, use and run for personal, non-commercial use, one (1) copy of [Lion] directly on each Apple-branded computer running Mac OS X Snow Leopard or Mac OS X Snow Leopard Server...that you own or control.” In other words, if your Mac shipped with Snow Leopard, you can install Lion on it. If your Mac shipped with Leopard, but you later purchased Snow Leopard for, and installed it on, that Mac, you can install Lion on it. If you didn’t purchase Snow Leopard, you can’t install Lion.
Those situations are pretty clear. But what if, for example, you’ve got a family-pack license for Snow Leopard, and you’ve got a Mac that shipped with Leopard but that you never upgraded to Snow Leopard? Assuming that Mac is compatible, the Lion license agreement says you can’t upgrade to Lion until you first install Snow Leopard.
(This is just one scenario—I can think of a number of situations in which you might have Leopard on a Mac or an external hard drive, along with a valid license for Snow Leopard, and you'd rather not take the interim step of installing Snow Leopard just to upgrade to Lion. Having performed this two-step upgrade many times while researching our various Lion-installation articles, I can tell you that it’s a real hassle.)
But lets take a step back. While the letter of the law says you need to install Snow Leopard before installing Lion, the spirit of the law seems to be that a particular Mac just needs a license for Snow Leopard before you can install Lion on it. In other words, in our view, you should be well within your rights to install Lion on any of your computers for which you have a valid, current Snow Leopard license—even if you don't install Snow Leopard on it first.
So then the question becomes whether there are any technical reasons you can’t install Lion over Leopard. Based on our testing, the Lion installer refuses to install Lion onto a drive containing Leopard (10.5); in fact, it refuses to install on any drive running a version of Mac OS X below 10.6.6. It will, however, install onto a blank drive, so Lion clearly doesn't need any of Snow Leopard's files or settings.
You may be thinking, “It will install onto a blank drive? Then I’ll just copy the installer to my Leopard-equipped Mac, connect an empty hard drive, install Lion there, and then use Migration Assistant to move my files over to it.” Alas, while the Lion installer will freely install Lion onto a blank drive, the installer itself must be run from within Snow Leopard or Lion.
So how can you install Lion over Leopard? There are three ways: the official way, the brute-force method, and the quick-but-techie way.

The official way

As I explained above, Apple’s official policy is that if you want to install Lion onto a Mac or a hard drive containing Leopard—assuming, of course, the Mac in question meets Lion’s system requirements—you must first intall Snow Leopard and then install Lion. This works, it’s fairly easy to do—if a bit time-consuming—and it gets the Apple seal of approval.

The brute-force method

What if you don’t want to install Snow Leopard first, or if you don’t have your Snow Leopard disc handy? (I’m not being coy here—perhaps you’ve misplaced it, or maybe you’re on the road and you’ve got your Mac’s original [Leopard] disc with you as an emergency boot disc, but you don't have your Snow Leopard upgrade disc.)
As I mentioned above, the Lion installer will let you install Lion onto a bare drive when the installer itself is run under Snow Leopard or Lion. So as long as you have a good backup; a 4GB-or-larger thumb drive or external drive; and either access to a Mac running Snow Leopard or Lion, or an already downloaded copy of the Lion installer, you can perform a bit of installer razzle-dazzle. You just erase your Mac’s drive, install Lion onto it, and then import all your data from your backup. (If this sounds a lot like a clean install, that's because it's essentially the same process.) Here’s are the steps to take:
  1. Make sure you have an up-to-date backup—either a Time Machine backup or a clone backup using a utility such as SuperDuper or Carbon Copy Cloner—of your Leopard Mac’s hard drive. (For this purpose, I recommend a clone.) Be sure to test this backup to verify that it has your latest data: In the case of a Time Machine backup, try restoring some important data from the backup; in the case of a clone backup, boot from the clone to make sure it boots and that it contains all your data.
  2. Use the Snow Leopard or Lion computer to download the Lion installer from the Mac App Store. (If you’ve already got your copy of the Lion installer, skip this step.)
  3. Create a bootable Lion-installer drive using the instructions in How to make a bootable Lion install disk or drive.
  4. Boot your Leopard Mac from that new Lion install drive. When you do so, you’ll find yourself with a screen called Mac OS X Utilities with several options. (This is the same screen you’ll see if you boot your Mac in recovery mode.)
  5. Select Disk Utility and click Continue, then use Disk Utility to erase your Leopard Mac’s internal drive. To do so, select that drive on the left, click Erase on the right, choose Mac OS X Extended (Journaled) from the Format pop-up menu, and click Erase. Warning: This step erases all the data on your Mac’s hard drive, which is why you needed that backup!
  6. When the erase procedure is finished, quit Disk Utility to get back to the Mac OS X Utilities screen.
  7. Select Reinstall Mac OS X and click Continue to launch the Lion installer and install Lion on your Mac’s internal drive.
  8. After your Mac restarts, installation finishes, and you proceed through the setup process, watch for the Transfer Information To This Mac screen. You’ll use the third option, From Time Machine Or Another Disk, to transfer all your files from your backup to your new installation of Lion.
When the transfer process is finished, you’ll be able to log in to Lion with all your accounts and data intact.

The quick-but-techie way

If you’re comfortable diving into the OS and editing a plist file, this is the fastest way to install Lion over Leopard, although, as with the previous method, you’ll need to be able to boot from a Snow Leopard or Lion drive to run the installer.
As I mentioned above, the Lion installer refuses to install the OS on a Leopard Mac. But how does the installer know your drive contains Leopard and not Snow Leopard? It turns out that the installer simply checks a particular file—/System/Library/CoreServices/SystemVersion.plist—on the destination disk to check the version of OS X currently installed on that disk.
Which means that if your Mac is running Leopard, and you’re feeling adventurous, you can edit the SystemVersion.plist file so that it claims you’re running, say, 10.6.7. The Lion installer—which will still need to be run on a Mac running Snow Leopard or Lion—will then install Lion over Leopard without the slightest complaint. Here’s how to do that:
  1. On your Leopard-equipped Mac, navigate to /System/Library/CoreServices.
  2. Using a text editor that lets you enter an admin-user name and password to edit system-level files—such as the non-Mac App Store version of TextWrangler—open SystemVersion.plist.
  3. Locate the ProductVersion key (not the ProductUserVisibleVersion key). Just below that is a string of numbers indicating the OS version; for example, on a Mac running OS X 10.5.8, it will read 10.5.8.
  4. Change that number to 10.6.6 (or 10.6.7 or 10.6.8), save the file (providing your admin-level username and password when prompted), and then shut down your Mac.
  5. If you’ve created a bootable Lion install disk or drive, boot your Mac from that, proceeding from Step 4 in the previous section. If you don't have a bootable Lion installer, you’ll need to boot from a Snow Leopard or Lion drive and then run the Lion installer from there. (If your Leopard Mac has FireWire, you could instead put the Mac in FireWire Target Disk Mode and then connect it to a Mac running Snow Leopard or Lion and run the installer from that computer.)
Whichever approach you take, when you’re done, you’ll have Lion on your previously Leopard Mac.

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