Salut, en fouillant un peu j´ai trouvé ça
Q: The email address assigned to my Xbox LIVE account is different to the email address I use on my PC for Windows LIVE messenger. Is there a way for me to still access my messenger account?
A: Not at this time. We are working on the ability to allow users to change the email address they originally used when signing up for Xbox Live. The feature will be available in an upcoming release.
En gros ça dit qu´il vont bientôt arranger cela, cependant aucune date n´a été anoncée
Source - http://www.majornelson.com/