Purchase a new subscription using an alias as primary apple ID.
Already have an appleID under which I have an alias - I want to use this alias and purchase a subscription so that me and my partner have two separate accounts. How do I do this?
MacBook Pro, macOS High Sierra (10.13.3)
If it is an alias of an Apple ID you are sharing, that won't make 2 separate accounts because it is tied to the same Apple ID. If you are sharing an ID, you might want to consider creating a new ID, and then set up Family Sharing.
There may be some data loss, but you can minimize it. If syncing photos with iCloud, ensure Optimization is turned off in Settings/Your Name/iCloud/Photos. Then make sure full resolution photos are on the device. When signing out of iCloud, if asked whether you want to retain data, choose yes. Messages conversations will be lost if they are addressed to the ID. See the article below for other places that the old ID should be removed. Also set up Family Sharing to keep access to your apps. Also see Winston Churchill's response in the below discussion
How to transfer all files from one iCloud account to another
Apple ID email address or password - What to do after you change
Family Sharing - Use your own Apple ID for Family Sharing
Family Sharing -What types of content can I share?
Family Sharing - If you don‘t see your family‘s shared content
Family Sharing - purchases and payments
Family Sharing - Hide and unhide music, movies, TV shows, audiobooks, and books
Family Sharing – share iCloud Storage
Family Sharing - Share an iCloud storage plan with your family
How to purchase a new subscription using an alias as primary apple ID
