The Guardians Advanced Content patch has been scheduled for July 19th, with the servers scheduled to be offline through the 20th. 
 
Sega have announced that players who own AotI will need to delete it, and then re-download it from the XBL marketplace in order to play online.  So current players should delete their AotI install, then watch the website for details, then re-download the expansion when everything is back up.  Of course, for players who don't already own AotI, it has become free to download in the recent months.