Frequently the somewhat despondent response "you can't merge/transfer IDs" posted on this forum is deemed by some as the simple, complete and ultimate answer to your question. While it is true that there is no single merge/transfer button, and some things (notably purchases) cannot be truly transferred, you can still frequently share purchases between your accounts and have access to them from the account you wish to use. Furthermore, you can transfer most data. I offer a more optimistic and helpful reply. Read the links below for things you may need to do to transfer as much as is possible and to share some others. However, given the complex nature of this and having to use Family Sharing to access some items, I would seriously reconsider setting up a new account.
Apps and media bought with one ID are permanently associated with that ID. Look into using Family Sharing to continue to be able to use items purchased with one Apple ID with other Apple IDs, and for some data sharing features.
- Family Sharing - What is Family Sharing? - Apple Support
- Set up Family Sharing - Set up Family Sharing - Apple Support
Share folders with iCloud Drive --> Share files and folders in iCloud Drive on iCloud.com - Apple Support
Read How to use Shared Albums in Photos on your iPhone, iPad, and Mac - Apple Support about how to share photos.
The next section may sound tricky but it lets you merge some things on a device where you have already signed in with an old account but want to use a new account and incorporate data that is already on the device from the old account. Incidentally, the data will then propagate to other devices using iCloud with that new ID. (I haven't tried this with a computer but it may also work with a computer account signed into an old ID.)
If you have signed into the first ID on a device then fortunately when you sign out of that ID, do not erase the device, then sign in with a different ID, it asks you in most cases if you wish to merge the data from the two IDs on your device (toggle the ones you want to on). Say yes and later delete individual items from the old ID that you do not want. Note that only content physically present on the device like contacts, photos and videos will be retained. Meta data such as a game play scores that are tied to a specific ID or are only on a vendor's database remotely, will not be retained. Also turn off storage optimization before doing this. With Optimize Storage, smaller, space-saving photos and videos are kept on your device while all of your original, full-resolution versions are stored in iCloud. Make sure photo optimization is turned off so the full quality versions from the old account are actually on the device before merging the content into your new account. Manage your photo and video storage - Apple Support You can turn optimization back on again once you are fully using your new ID on the device.
Sign out of iCloud on your iPhone, iPad, iPod touch, Apple TV, or Mac --> Sign out of iCloud on your iPhone, iPad, Apple TV, or Mac - Apple Support
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' --> Sign in with your Apple ID - Apple Support
