I upgraded my laptop from Fedora 13 to Fedora 14 by preupgrad command. I've been using F14 in virtual environment and I pretty sure upgrading my laptop won't have serious problem because of upgrading. Although, it's quite big to upgrade because it's not a test machine I use it everyday :)
So, upgrade your system by preupgrade package is quite easy. You need to follow this test case's steps.(QA:Testcase_Preupgrade)
When preupgrade command is finished, you need to reboot your system. By the way, if you reboot your machine, it take sometime to see boot messages. so you'll see just black screen for a while but don't worry. It'll start anaconda soon. Then anaconda starts installing packages. When anaconda finishes package installation and upgrade process, it'll reboot your machine. then you'll be able to login new Fedora Linux.
When I logged in, I need to upgrade packages. I had about 200 packages to update.
During upgrade process, I didn't see any problems also I haven't seen critical problems in casual use e.g. use wifi connection by WPA2, run firefox, watch youtube, writing Japanese, send and receive email by thunderbird, run command from gnome terminal and so forth.
Finally, as I proventester, I have good opportunity to test Fedora 14 :) you know, I'm going to use it everyday, I'll have a chance to find bugs!
