As I mentioned before, 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.