FOR THOSE WHO’VE used Apple’s Migration Assistant to move everything from an old Mac to a new one, all your apps, customization and settings should have migrated too — so you’ll have the correct account details in Mail, your purchased content in iTunes or Music, and messages up and running.
If you’re already a Mac user, chances are you’re using iCloud for a lot of things: synchronizing Safari bookmarks and settings, contacts, calendars,
