You could upgrade from NexusHD2-JellyBean V1.4a. (Remember to backup first.)
However a fresh install is recommended.
When doing a fresh install and logging in your Google account, some of your configs (e.g., Wi-Fi) and previously installed apps could be restored from the Internet from Google automatically.
Thus don't play your HD2 too soon after first boot (maybe wait 10 mins).
Then you will have a fast and stable CM 10.1.
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