[Darryl Ramm]

Why do you think you need to downgrade? What exact problem are you trying to solve? What are you reading exactly (link please) or who is telling you that you have to do this? (You might well have to, but don't assume you have to just because stuff says it's not officially supported). And doing minor OS X upgrades can always break things so i want to make sure you have done basic troubleshooting and/or are getting good advice on the need to do this... like from the good folk on DUC, not Apple support.

What exact date code/name or model number is your Mac?

What exact OS X version did it come with? If you remember?

If you upgrade and want to roll back, time machine is *not* what you really want. Not at all (but it can work if all you have). You really want an image copy/clone of your boot disk made with something like Carbon Copy Cloner. If doing that you would just boot off the external disk drive your clone copy is on and see if that works, that test is as fast as rebooting the iMac. If you have not been making backups well... go buy a 7200 rpm external HDD (USB3 is great, G-Tech or similar quality brand) and a copy of Carbon Copy Cloner and work out how to use it, and start doing so as soon as your system is set up properly (maybe before if you want to move stuff over).

If you send your computer out to a third party you might as well clone it with Carbon Copy Cloner now and use that to recover stuff later. Otherwise as Matt says you will lose everything on the computer. I also agree if your computer can run Yosemite (came with Yosemite) then I would install that. But first I'd really focus on if you need to roll back at all, not being supported congifs, and being broken now does not mean your next step is to rollback, it means debug stuff, unless you have hard advice that this will absolutely not work.