As part of Brussels Airlines full integration into Lufthansa’s Miles and More program, Privilege members who are carrying a mileage balance must register or link their account to Miles and More by July 31 to keep ensure those miles are equally converted (they guarantee this). It is not too clear what happens if you don’t do it by July 31 but it looks like they may not guarantee that some or all of your miles will convert to Miles and More.
To complete the migration by July 31 visit the following page on Brussels Airlines website:
http://www.privilege.brusselsairlines.com/migrate/