So then the question becomes whether there are any technical reasons you can't install Mavericks over Leopard.
In my testing with many Macs, the Mavericks installer, like the Mountain Lion and Lion installers before it, refuses to install onto a drive containing Leopard; in fact, it refuses to install on any drive running a version of Mac OS X below 10.6.8, just as its official system requirements claim. The Mavericks installer will, however, install onto a blank drive, so Mavericks clearly doesn't need any of Snow Leopard's files or settings.
You may be thinking, "If it will install onto a blank drive, I'll just copy the installer to my Leopard-equipped Mac, connect an empty drive, install the new OS there, and then use Setup/Migration Assistant to move my files over." Alas, while the Mavericks installer will let you install the OS onto a blank drive, the installer itself must be run from within Snow Leopard, Lion, Mountain Lion, or Mavericks.
So how can you install Mountain Lion over Leopard? There are three ways: the official way, the brute-force method, and the quick-but-techie way. Whichever method you choose, you should--as with any OS installation--be sure to have an up-to-date, tested backup of your drive before you begin.
Note that there are actually two Mavericks-compatible Macs--the Mid 2007 iMac and the Mid/Late 2007 MacBook Pro--that shipped with Tiger [Mac OS X 10.4]. If you've got one of these Macs, still running Tiger, and you're determined to upgrade it to Mavericks, the first two methods below ("The official way" and "The brute-force method") will work; the third method ("The quick-but-techie way") will not.
The official way
As I explained above, Apple's official policy is that if you want to install Mavericks over Leopard--assuming, of course, the Mac in question meets the system requirements--you must first install Snow Leopard, purchasing it for $20 if necessary, and then install Mavericks. This approach works fine, 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 the disc, 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 Mavericks installer will let you install onto a bare drive as long as the installer itself is run under Snow Leopard, Lion, Mountain Lion, or Mavericks. This means that as long as you have a good backup; a 6GB-or-larger thumb drive or external drive; and either an already-downloaded copy of the Mavericks installer or access to a Mac running Snow Leopard, Lion, Mountain Lion, or Mavericks, you can perform a bit of installer razzle-dazzle.
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