So, you’ve got a brand new IMac or Mac Mini or Mac Pro and it’s come with the latest version of the OS – 10.9. The problem is that you have productivity software that doesn’t play well with 10.9 and you want to bring the machine back to OS 10.8. Possible to do? The answer is “yes,” but you stand a very good chance of turning the machine into a shiny aluminum brick if you’re not careful.
The first thing to do is to go into the Apple Menu -> About this Mac. This will tell you what version of the OS you are currently running. Clicking the “More Info” button will get you more detailed information including when the machine was released iMac (27-inch, Late 2013). Clicking on the System button will provide a two paned window. Clicking on “Software” in the left-hand pane will give you information about the version of the system that the machine has (“System Version”) as well as the build of the OS (usually starting with the numbers 12 or 13).
Apple makes it a point not to make the driver infrastructure for new hardware to be made available to older versions of the OS. That said, if it’s a Late 2013 iMac, according to Apple, it shipped with a custom build of 10.8.4 (how Apple usually phases in new hardware to current releases). If you can find that build, 12E4022, it should run just fine. Not an easy thing to do, however. MIT doesn’t keep a library of custom builds since they are only released on new hardware.
Also, In Theory, the unified build 12F37 for 10.8.5 should also work, but we don’t have any confirmation on that.
For reference: Here is a list of custom builds for mid lifecycle released hardware: http://support.apple.com/kb/HT1159 And here is an overall list for 10.8 builds, with distributed software updates and also those custom builds for hardware. http://en.wikipedia.org/wiki/OS_X_Mountain_Lion#Release_history
All in all, the effort may not be worth it. You should investigate all work arounds before attempting anything like this.