We had an older 21" iMac bite the dust so we purchased a new 27" late 2013 build iMac but due to incompatibilities with Esko & HP DFE (Indigo) software we're not able to run Mavericks which came on the machine and needed to move backwards to Mountain Lion.
Apple has made it next to impossible to go backwards with the OS and it's been a total pain in the butt!
The process went very well and the directions from iFixIt were spot on! One word of advice... the new iMac is using "flash" storage not an SSD so there was really no need to open the unit.
But the process of cutting it open and sticking it back together isn't difficult at all, so don't be afraid to try it if you want to replace the hard drive on the basic machines that use them.
THE FIX: I used a hard drive dock with a spare drive I had on hand and install OS 10.8.5 on it. Then I booted the new iMac from that hard drive via the usb hard drive dock.
NEXT STEP: I used the Disk Utility app and erased the Mac HD flash storage, and then used my Mountain Lion thumb drive and installed the OS on the Mac's flash storage. The entire process took about 30 mins. and the iMac had zero problems running the older OS.
LAST STEP: I did a migration from the old 21" iMac using a Time Machine backup and "bam" all is right in my graphics world once again.