Will my old iPhone retain all content after I migrate to a new iPhone?
Will my old iPhone 6 retain all it's content after I migrate to a new iPhone SE 2020? Can I use it as a back-up to my new phone?
iPhone 6, iOS 12
Question marked as
Yes, you can use the old phone for a backup phone. As long as you don’t erase the phone, all of your content will still be there. You should backup your old phone so that you can restore the backup to your new phone.
Transfer data from your previous iOS device to your new iPhone, iPad, or iPod touch - Apple Support