Last week I downloaded Windows Mobile 6 for my HTC TyTN. Today I actually went ahead and installed it.
First of all I decided that the only important data that was worth backing up was my contacts list. It turns out that all contacts are stored in a file called pim.vol which can easily be copied to the desktop using the standard ActiveSync tool. So no special software was needed at all.
Next, I ran the executable that I downloaded last week, stepped through the wizard, waited for 15 minutes and... voila... Windows Mobile 6 was up and running.
After the installation, I copied the pim.vol file back onto the device and all my contacts were back. I also enabled my ringtone again without any problem.
I still have to reinstall the original copy of Route 66 that came with my device and a small number of other useful utilities. But right now I'm already happy that the upgrade itself went without any problem.
Correction: Copying the pim.vol file did not work as expected because after rebooting the device I got a whole bunch of old appointments (not that bad) and I could no longer start the contact application (really bad). In the end I decided to rename the pim.vol file, reboot and let it create a new blank pim.vol file. Most of my contacts were still on the SIM card anyway.