I stupidly downloaded the new OS X Yosemite 10.10 on my 2011 iMac that does all of my Pro Tools work and now some of the things I use aren't compatible yet. So I'm trying to figure out how to downgrade back to OS X Mavericks 10.9. I also stupidly don't have a Time Machine Backup or this would be way easier.
I learned you could install the old operating system with a clean install from a bootable drive with the installer on it, which I've figured out and made, but I don't know if there is a way to install it with wiping all the data on my system hard drive. I read you can transfer files back and forth from a Time Machine backup, but I can't seem to find out if I make a backup of my computer as it is, now with the current OS X 10.10, would those files and applications be transferrable to a clean install of OS X 10.9.
If there is a way to use my bootable install of OS X Mavericks and have it replace OS X Yosemite, and to keep all my files on the hard drive, that would be easiest.
Best Answer
Honestly I would make a clone of your current drive onto an external HD. Then boot from it to make sure it works.
Once that is done I would boot from your Mavericks install key (unplug your backup drive!), reformat the drive and install Mavericks. Once that is done boot to the fresh install and do all of your updates so you have the latest version of 10.9.
I would recommend doing all of this from a username/account that is different from the one you normally use. Migration Assistant will re-create your old user account later.
Re-install your applications (I would do this as there may have been Yosemite-specific) updates that were applied to your apps that Mavericks might not like).
Then use Migration assistant, point it to your backup drive and have it migrate your documents and settings to the fresh install of Mavericks. I would avoid migrating the apps for the reason above.
Now, hang onto that backup drive and make another clone before you do any point (10.10 to 10.10.1) or major upgrade (10.9 to 10.10) to your OS, so you can revert more easily in the future. I always do it, yeah it slows down the upgrade, but speeds up fixing the upgrade if it breaks your software.