The iCloud Music Library (iCML) is by definition tied to an AppleID. Switching AppleIDs on a device (or in iTunes) obviously has to replace the Apple Music music (ie, music obtained through the Apple Music subscription as opposed to purchased, ripped or otherwise obtained music) on the device or in iTunes with the one from the iCML associated with that new Apple Music. That is the whole point of the iCML, you can log in anywhere and have access to all 'your' music. As long as there is an unmetered internet connection and music can be streamed that is not a big problem. But for offline music that presents a major headache. At best Apple Music gains an easy method to re-download a complete library (or an easily marked subsection of it). On a computer with a broadband connection that could be acceptable. But already downloading a dozen or more GB of music regularly via WiFi on your phone is somewhat of a hassle. For those who are wondering, users of multiple AppleIDs (at least if those are from different countries) need to login to each of their respective AppleIDs to get updates for the apps bought with them. And that is before other aspects like wanting to use a particular AppleID because something is only available in one country (or is noticeably cheaper in one country). Am I missing something here or has anybody an idea how the iCML could ever easily co-exist with using multiple AppleIDs?