I set up my new iPhone 12 from my old iPhone 8. Now how do I shut down and wipe off my old iPhone 8 without losing everything I just transferred to the new iPhone 12?

Why do you think erasing the old phone will wipe out the new one?

If you are planning on getting rid of the iPhone 8 follow this: What to do before you sell, give away, or trade in your iPhone, iPad, or iPod touch - Apple Support

On your iPhone 8-

Open Settings ➔ General ➔ Reset ➔ Erase All Content & Settings : This will sign you out of Find My, your Apple iD (iCloud) and reset the iPhone back to factory settings. You will not lose any data on your new iPhone as a result of this action.

Axel F.

Hi Deggie,

I just did the above (Erase all contents and settings) on an iPad (a few days ago) and have done this several times on iPhones over the years. The process Turned OFF Find My, The process has always wiped the device clean and returned it to factory settings and at the Hello Screen. The device was also removed from under the device list for that Apple ID. What am I missing?

Axel F.

It doesn’t sign you out of FaceTime, IMessage or iCloud and I’ve never seen it turn off Find My. A lot of posts here are from people who erased their iPhone, sold it and then the buyer ran into issues setting it up. I always follow the steps in the link.

Axel, that will not log you out doing that, read the link I provided.