Get old iTunes transferred over to my new account
How can I get my old iTunes music transferred to my new account
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Merging Apple accounts (IDs) or transferring data is not officially supported, nor can it be done easily or completely.
Apr. 2018 post by ASC helper Winston Churchill on transferring data from one Apple ID to another. https://discussions.apple.com/message/33333380#message33333380
Feb. 2021 tips from Eric Root https://discussions.apple.com/thread/252468781?answerId=254653802022#254653802022
Look into using Family Sharing to continue to be able to use items purchased with one Apple ID with other Apple IDs.
Family Sharing - https://support.apple.com/HT201060
Set up Family Sharing - https://support.apple.com/HT201088
Fortunately when you sign out of one ID and sign in with a different one it asks you in most cases if you wish to retain the data from it on your device. Say yes and then delete individual items you do not want.
Sign out of iCloud on your iPhone, iPad, iPod touch, Apple TV, or Mac --> https://support.apple.com/HT208242
If another Apple ID is signed in you may need to enter the password for that ID to sign out of that ID first.
Click on this link to learn how to 'Sign in with your Apple ID' --> https://support.apple.com/HT204053
For media, make sure you have downloaded everything to your computer and make sure it is backed up.