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 despite the team working on it all weekend, 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.