what happens if i change the apple id on a family iPhone

When you change the Apple ID that you use for Family Sharing, your family members can't access or download purchases from your previous account. They also can't use any DRM-protected music, movies, TV shows, books, apps, or in-app purchases that they previously downloaded from that account.

If this is a phone the family shares and not family sharing, changing the Apple ID may cause some data loss.

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. 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. See Winston Churchill's response in the below discussion.

 How to switch Apple IDs on your iPhone or iPad.      

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

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