I started a new iPhone from last Saturday, and I formatted my previous iPhone two days ago.
After transferring the setting of the old iPhone, we must be careful if we are not going to use it any longer.
Especially in case we are planning to sell our old phone to a secondhand electronic appliance shop, it must be formatted beforehand.

It took me about two hours to format my old iPhone 4S.
The reason was I forgot to disconnect my old iPhone from Find My iPhone app at first.
The important step for disconnecting it was to delete the iPhone I would quit using from the application.
After that, I could delete the old iCloud settings and format the phone very easily.

I didn't decide to sell my old iPhone to a secondhand shop but gave it to my youngest daughter, who is a feature phone lover.
She says, "A simfree iPhone is useful enough for me to play game apps and have talks on Skype.
She is looking forward to talking with her sister in Tokyo on Skype with her used iPhone.