One feature you may not even be aware exists is the ability to change your Windows Live ID e-mail address and have all your associated accounts and contact list automatically updated. This isn’t a new feature by any means, but it’s always been placed subtly in the Live ID/Passport web site so most people wouldn’t discover it unless you were really looking for the feature. If you want to see where it currently is, sign into the Windows Live ID account site, and just note the ‘Change’ link next to your e-mail address.
Jonathan Kay is warning users not to change Windows Live ID right now. Reason: this feature has become unreliable and in general, your Messenger contact list will never be updated.