Apple has pitched the £25 single-licence Snow Leopard upgrade only to customers running Mac OS 10.5, known as Leopard. At the same time, it's told users running the older Mac OS 10.4, known as Tiger, that they should purchase the more expensive £129 "Box Set" instead. The Box Set includes the Snow Leopard upgrade, and adds the iLife '09 creativity bundle and the iWork '09 productivity suite.

But acccording to Walt Mossberg, the technology columnist for the Wall Street Journal, Tiger users could opt for the cheaper upgrade. "Apple concedes that the £25 Snow Leopard upgrade will work properly on these Tiger-equipped Macs, so you can save the extra £104," Mossberg said.

Answers to important questions are scarce, however, including whether Tiger users would be required to do a so-called "clean" upgrade that involves backing up their data, erasing the hard drive, installing Snow Leopard, then restoring their data and reinstalling all applications. (Snow Leopard's default for Leopard users is an "in-place" upgrade that retains all data, settings and applications.)

If Tiger users must conduct a clean install with the £25 upgrade, they would be in the same boat that Windows XP users will be come 22 October when Microsoft launches Windows 7. The only way to upgrade from Windows XP to Windows 7 is to do a clean install. Only users running Vista can upgrade in-place.

Others confirmed Mossberg's note that Tiger users can go the cheaper route. "Our tests confirm 10.6 DVD works for Tiger Macs too," wrote Brian Chen, a reporter for Wired.com on Twitter. "Apparently it installs even if you have Tiger."

Apple did not reply to questions today.

One thing is relatively certain, however: Using the £25 Snow Leopard upgrade on a Mac running Mac OS X 10.4 is a violation of Apple's End User Licence Agreement, or EULA.

"If you have purchased an Upgrade for Mac OS X Leopard licence, then subject to the terms and conditions of this License, you are granted a limited non-exclusive license to install, use and run one (1) copy of the Apple Software on a single Apple-branded computer as long as that computer has a properly licensed copy of Mac OS X Leopard already installed on it," the EULA reads, according to TidBits' Joe Kissel, the author of Take Control of Upgrading to Snow Leopard.

Apple is selling single-licence Snow Leopard upgrades for £25, and a five-license Family Pack for £39. The Box Set costs £129 for a single licence and £179 for a five-licence pack.