Restoring Preferences after downgrade from El Capitan to Yosemite I had to revert from 10.11.5 to 10.10.5 because of some devices not working on the newest OS. I had prepared a USB flash drive a while ago and in order for it to work I had to erase the partition the OS was on. I had been doing Time Machine backups regularly and before erasing the hard drive I copied the Users, Library and Applications folders to an external hard drive. Now I have a clean copy of 10.10.5 installed and transferring some of the files wouldn't work out because of the different operating systems. My question is if there is a way to keep my 'old' system preferences(at least) without having to set everything up manually.
OS X stores application preferences in a few locations

Restore the files and folders from these locations in your backup:
    /Users/username/Library/Preferences
    /Users/username/Library/Application Support
    /Users/username/Library/Containers

To the corresponding locations for your new user account.
No guarantee though that the plist files from the newer OS X system version will work in the older one though. Try it.