US date format version Rated 4 out of 5 stars
It won't fix your old xnotes, but all new xnotes that you add will be in the US date format. MM/DD/YY
Also, if you update the new US Date plugin, it may revert back to the old date format for new xnotes (if the author doesn't change it on his end).
No need to uninstall, just install right over the top. It won't affect your old xnotes if you use the same version (2.1.0).
If you want to fix your old notes, go here...
C:\Documents and Settings\*USERNAME*\Application Data\Thunderbird\Profiles\*RANDM-CHARACTERS*.default\XNote\
You should "probably" be ok to edit the old xnote files Notepad, but hex-edit would probably be safer.