I've only hit one memorable problem when upgrading Office.
That was in Office 2007, when MS replaced the 'File' menu option with that Office logo button.
I installed 07 to get familiar with it. Soon I wanted to save and print a spreadsheet, but I couldn't find a readily apparent way to perform those tasks. Instead of immediately hitting the F1 key (another tool users aren't aware of or have forgotten), I decided to see how long it would take to figure it out on my own. After 30 minutes, I gave up, pushed F1, and learned that what I thought was just a logo was actually a clickable button. I made a note to point it out to each user as I upgraded his / her suite.
Apparently I wasn't the only one with this problem, since it was gone and 'File' was back in the 2010 version. Sometimes MS does realize it made a mistake. (Here's hoping they give Metro the same treatment in W9.)
There are sometimes pages on the MS site how to perform familiar tasks in the newest version of their applications.