Finally forced to upgrade to Yosemite but worried I might lose files

It won’t unless a problem occurs.

(132445)

It does not delete nor touch your files. Be sure you have no problems on the current system. If you do get those fixed first. Be sure you repair your hard drive and permissions using Disk Utility before upgrading. You are smart to have backed up your files.

Repair the Hard Drive and Permissions

Boot from your Snow Leopard Installer disc. After the installer loads select your language and click on the Continue button. When the menu bar appears select Disk Utility from the Utilities menu. After DU loads select your hard drive entry (mfgr.'s ID and drive size) from the the left side list. In the DU status area you will see an entry for the S.M.A.R.T. status of the hard drive. If it does not say "Verified" then the hard drive is failing or failed. (SMART status is not reported on external Firewire or USB drives.) If the drive is "Verified" then select your OS X volume from the list on the left (sub-entry below the drive entry,) click on the First Aid tab, then click on the Repair Disk button. If DU reports any errors that have been fixed, then re-run Repair Disk until no errors are reported. If no errors are reported click on the Repair Permissions button. Wait until the operation completes, then quit DU and return to the installer.

If DU reports errors it cannot fix, then you will need Disk Warrior and/or Tech Tool Pro to repair the drive. If you don't have either of them or if neither of them can fix the drive, then you will need to reformat the drive and reinstall OS X.

Quit all running applications. Disconnect all peripherals except the Apple mouse. I would even go so far as to suggest booting the computer into Safe Mode, then restarting normally just prior to running the Yosemite installer or you can run the installer straight from safe mode.