Apple ID

When changing to the new ID, 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 and/or in Photos/Preferences/iCloud. Then make sure full resolution photos are on the device/computer. 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. See Winston Churchill's response in the below discussion. If possible, also set up Family Sharing  between the IDs to keep access to your apps/purchases. 

 How to switch Apple IDs on your iPhone or iPad.      

How to transfer all files from one iCloud account to another.       

What to do after you change your Apple ID or password - Apple Support.      

Family Sharing.         

Family Sharing - Set up.        

Sjs5353 Said:

"Apple ID: I have two iPads and one iPhone all using the same Apple ID. I want to establish a new Apple ID on the iPhone and one iPad. How do I do it?"

-------

Sign Out:

Sign out of Apple ID on the iPhone, and then sign in using the other Apple ID. Note that you may not be able to do this, as you have to wait up to 90 days to associate a device with another account. So, see if you can perform this Sign out/ Sign in.