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I think HTC changed the URL. I've edited the link in the above post - try it now!
The main difference is that it upgrades the operating system from Windows Mobile 6.1 to Windows Mobile 6.5, which is a bit more "finger-friendly".
And no, it will completely erase everything in the phone's ROM, so all customisations, installed apps, etc. will have to be redone from scratch. It won't damage any data on the memory card (but apps installed to the memory card will probably need to be reinstalled).