OS X is changed, but not users files. However the install can go very badly, and without a back up, cost a lot of money in data recovery efforts.
Always backup your users files to external drives before inflicting any major change or do so routinely to counter if the machine fails to function out of the blue.
Everything in software can be replaced, except your users files as they are unique and personal.
I had a boot hard drive just suddenly not work one morning. Dead as a doornail.
Only took a few seconds to boot from my bootable clone and call Apple for a replacement drive.