There seems to be a bug in the new 360 Dashboard update related to changing your Live ID.
According to a post by a forum staff member at the xbox.com forums, if you change your Live ID, which is linked to your gamertag, you’ll lose the ID on all your forum posts, CUL and icons associated with your gamertag. All your resulting messages on the xbox.com forums will read as "anonymous."
Here’s the post from the forums:
"If you change the Live ID associated with your gamertag (as the Dash update allows you to do), you will lose all of your forum posts, CUL and icons. All your posts will appear as ‘anonymous’. We’re hard at work on a fix, but we don’t have a fix ready to go out with the update.
"If you are particularly attached to your Community User Level or forum icons please DON’T change your Live ID until we deploy a fix. It should only be a few days, and we apologize for the inconvenience. We will let you know as soon as it is safe to change your Live ID and keep all your forum goodness.
"To clarify: this only affects those who are attached to their Community User Level on the forums and do not want to lose any associated icons or level. Any other links with your Windows Live ID on your console and Xbox.com should be okay.
"If you don’t care about your forum posts, CUL or icons, then hey…enjoy the new feature. Otherwise, do not change it until a fix comes out so your CUL, icons, posts can stay in tact."
So if you have thousands of posts that you hold dear to your heart and want the world to know just who was behind those brilliant messages, make sure you don’t change any of your stats until the fix is out.
