I tried to downgrade to Mavericks yesterday, it was almost impossible. The installer refuses to wipe clean and install a lower version unless you do some magic to the disk first and use web install. It took over 5 hours and I downloaded Mac OS 4 times from different sources, including Apple.
The ram it on our throats with free upgrade, then block our path to downgrade, unless we have a time machine backup to restore from. I didn't.
My wife and I have identical MBAirs, except that I got mine when it was shipping with Mavericks and she got Yosemite. I get about twice the batter life, and a lot less trouble with Pages/Numbers.
I'm thinking of wiping hers and installing from TimeMachine, then pulling in just her user data.
The ram it on our throats with free upgrade, then block our path to downgrade, unless we have a time machine backup to restore from. I didn't.