SAME APPLEID ON 6S PLUS AND 7 PLUS

Please don't type in all capital letters. It makes your post harder to read and is considered shouting on the Internet.

You should be able to change it on one phone, but not the other.

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. 

Apple ID – Create

Apple ID - create a new

How to transfer all files from one iCloud account to another

 Apple ID email address or password - What to do after you change

Family Sharing

Family Sharing - Set up