Once you install Yosemite, the only way you'll be able to revert to Snowy is with a clean install...nothing new, by the way (*), which means you'll lose everything on the disk, so you must do some sort of backup before you upgrade.
You're obviously not running Time Machine, but are you running any sort of backup software like Carbon Copy Cloner or SuperDuper?
What sort of backups do you do now, and to what sort of device?
We can proceed from the answers to those questions.
Edit: (*) I don't think OS X has ever allowed the installation of an earlier version over a later one.
Last edited by artie505; 02/06/15 03:20 AM.
