Migrating to C6 - Old Sharing

Migrating to C6 - Old Sharing

I want to migrate to Creator 6 but it says I'm ineligible. I've narrowed the reason down to:

Old sharing
"Some of the applications in your account uses old sharing (C4) which cannot be migrated to the new sharing. Migration of these old sharing into the new version is a work-in-progress. The workaround would be to delete the old sharing manually and proceed for upgrading."

But I don't know what that means. Can someone point me in the right direction? Is this referring to app sharing? User permissions? What do I need to adjust?