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Change OS X Version
I suppose the question would arise, “Why would one want to do this?” On Leopard, it is incredibly useful in situations where one may wish to install software that claims to require an update of OS X that you don’t want to install or you can’t install. Or some apps don’t support the newest OS X. I’ll be demonstrating on Snow Leopard, but the procedure is the same on all OS X.  Please read the whole post before changing the version of your OS X.
How to change the OS X Version?
  1. First go to “/System/Library/CoreServices/” and make a copy of  “SystemVersion.plist” on your desktop and rename it to “SystemVersion_bak.plist”
  2. Second open Terminal.
  3. Type “sudo nano -w /System/Library/CoreServices/SystemVersion.plist” (Without the quotes) and press enter.
  4. You will be prompted for your password, type it in and press enter.
  5. Use the arrow keys to navigate to the end of the 10.6.2 under “ProductUserVisibleVersion” and erase the version number.
  6. Enter in a version number of your choice.
  7. Repeat for the version number displayed under “ProductVersion”.
  8. Press Control-X to exit.
  9. It will ask you if you wish to “save modified buffer.” so press Y.
  10. It will ask for a filename to write. Press Enter to accept the default name.
  11. You’re done!! :)
Screenshots
About_This_Mac-20100210-193506  Terminal_—_bash_—_80×24-20100210-193625Terminal_—_bash_—_80×24-20100210-194009Terminal_—_bash_—_80×24-20100210-194247Terminal_—_bash_—_80×24-20100210-194604 About_This_Mac-20100210-195123
Notes
Never restart your computer while you have changed the OS X version or else after the restart most of the apps and services won’t work. Which means always change the version back before restarting/shutting down your mac. If that happened by mistake just re-name  “SystemVersion_bak.plist” that you copied to your desktop to “SystemVersion.plist” and replace it with the file in “/System/Library/CoreServices/” and restart your computer.
There are some apps that ask you to restart the computer after it has finished installing. Just use the tutorial above and change the version back to the original one, save it, then restart your computer.
There are lots of apps that work only by changing the version of OS X you are using, but you may also come across apps that still won’t work just by changing the OS X version so keep that in mind.