Apparently the update allows you to change the email account (Live ID) associated with your Xbox Live GamerTag, which is a welcome feature, and one that has been requested since the console launched.
Right now the problem affects the Xbox.com forum. If you change your email, you lose all of your posts (they show up as 'anonymous'), as well as community user level, and icons.
The fix should be out in the next few days, so if you value your CUL, icons, and posts, don't change your email until then.