How can I switch phones with a family member without losing data on either phone?

jkatts wrote:

How can I switch phones with a family member without losing data on either phone?

Make a backup of each phone to iTunes or iCloud (be sure to use your personal Apple ID or computer login), then do whatever your carrier wants you to do to switch numbers if that's your intent, then restore those backups to the 'other' device.

Just erase the device via settings or an iTunes restore after making the backup. If possible, do both an iCloud & iTunes backup. As long as you erase the device, none of your information should remain. All that will restore is the data associated with the backup.