I have had the same Appli ID as my wife for 10 years, but this gives more and more problems. I would now like to have my own apple ID, but my wife's data cannot be lost, nor can my data be lost. We just want to separate everything.
iPhone 6
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. Also set up Family Sharing to keep access to your apps/purchases.
How to transfer all files from one iCloud account to another
Apple ID email address or password - What to do after you change
AppleID change
