Downgrade Yosemite Release to Mavericks
Clone the drive to a completely separate device.
Setup Yosemite on one copy, keep the other to swap back if you don't like it.
Yosemite has been seen to change the drive structure from 'regular' GUID to Core Storage, giving people serious headaches trying to get back to the old setup.

Personally, I wouldn't touch Yosemite yet without this kind of fallback solution. 

I know effectively in SE I'm in the doctor's waiting room complaining it's full of sick people - but there have been too many 'oopsies' in the past week for me to suggest anything less safe.