You may lose some of your apps if they can't run under Mountain Lion.The installer may also change some of your preference files because certain settings are no longer supported or are incompatible with some feature of the new OS.On the plus side, Apple worked out most of the glitches on Lion, so you get the benefit of installing Mountain Lion using a well-understood and reliable method.If you did upgrade to OS X Lion, you'll find most of the installation process to be very similar.Before you install Yosemite be sure to check you have the right Mac.The following qualify: i Mac, Mid-2007 or newer, Mac Book, Late 2008 Aluminum, or Early 2009 or newer, Mac Book Pro, Mid/Late 2007 or newer, Mac Book Air, Late 2008 or newer, Mac mini, Early 2009 or newer, Mac Pro, Early 2008 or newer, Xserve, Early 2009.Make sure your Mac also has: OS X Mavericks, Mountain Lion, Lion, or Snow Leopard v10.6.8, already installed2 GB or more of memory8 GB or more of available disk space. Select App Store from the Apple Menu to open the App Store, then click the Download button for OS X Yosemite in the App Store window to begin installation.
Most of you won't have any problems with installing and using Mountain Lion, but there's a small chance that your particular combination of apps, data, and preferences will be the one that was never thoroughly tested before Mountain Lion was released.
That's one reason why I highly recommend backing up your current system before you start the upgrade process.
Either way, this step-by-step guide will help ensure that you understand how everything works.
The upgrade install process lets you install Mountain Lion over your existing version of OS X, and still retain all of your user data, most of your system preferences, and most of your applications.
This guide will show you how to perform an upgrade install, which is the default installation and the one that Apple thinks most Mac users will choose. You can also perform a clean install, or install the OS from various types of media, such as a USB flash drive, a DVD, or an external hard drive. OS X Mountain Lion is the second version of OS X that can only be purchased through the Mac App Store.
If you haven't yet upgraded to OS X Lion, the new distribution and installation methods may seem a bit foreign.