Changing of an Apple Id

Your data will not be lost if you proceed as below. See the instructions below.

Change your Apple ID (Be very aware of the restrictions here.) https://support.apple.com/en-us/HT202667

Before changing your Apple ID using a computer, sign out of all Apple  services that use your Apple ID. If you are going to use an iPhone, iPad, or iPod, then stay signed in with your Apple ID on only that device. This instruction is taken from the above document under the heading Before you change

your Apple ID.

For sign out purposes, see Where can I use my Apple ID? https://support.apple.com/en-us/HT202659“This list is provided for informational purposes only and is subject to change without notice. It's not intended to include every feature or service that uses Apple ID. Visit the Apple ID support page for more information.”

There may be some redundancy, but see What to do after you change your Apple ID or password https://support.apple.com/en-us/HT204071

The Apple ID that is signed in an iPhone will sync those photos when photos box is turned as on https://support.apple.com/en-in/HT204264 and will be visible in iPhone .

Before changing your Apple ID transfer them to Mac https://support.apple.com/en-in/HT201302

https://support.apple.com/en-in/HT204055

And then change Apple ID https://support.apple.com/en-us/HT202667

Read this also https://support.apple.com/en-in/HT204071