Upgrading an operating system is always a traumatic experience for a computer—and, too often, for its owner. If you can rein in your excitement long enough to perform just a few extra steps, you’ll be glad you did.
Check your hard drive. Open Disk Utility (Disk Utility), click your main hard drive’s name and then click Verify Disk. If the program reports that you have problems, you’ll have to start up from a different disk (like your recovery disk—The Homemade Installer Disk), run Disk Utility again, and use the Repair Disk button this time.
Turn off FileVault, if you’re using it. (You’d remember turning on this Home folder–encryption feature.)
Make sure your software will run. Most modern programs run very smoothly on Yosemite. But some older ones won’t run—in particular, programs like AppleWorks and Microsoft Office 2004 that relied on the old Rosetta translation technology. Find out for sure by visiting http://roaringapps.com/apps:table and looking up the programs you own.