I always wait a while before erasing my old iPhone, just in case I need something. But I'm thinking this is unnecessary, as I've never needed anything on my old phone. Once your new one has all data transferred over and is up and running, do you immediately erase your old one?
I got stung by that when I moved over from iPhone X to 14, the First Direct banking app informed me I needed to generate a security code using the app on my old phone to activate the app on my new phone...err whoops!
Think that is more poor app design than security though
This stuff is really annoying if you need a letter from a bank to reactivated some app on a new device for example.
I already purchased a photo-TAN generator and card reader for two banks. Also SMS-TAN is still activated where still possible.
And back to the main topic. I always keep a spare phone with everything working on it. Mostly it was a third older phone and then I sold my other one directly. But since I don't buy expensive new phones anymore. I just keep them all. There is always need for a fast replacement in the family and than it's good to have some cheap older phones lying around that still work perfectly.
P.S.: I wonder if those authenticator codes in the new Password app are transferred to new devices. I put all I had in there too.
Usually I'd do it on the Sunday or Monday after release day, minimum of at least 24 hours just to get it right.
Yesterday though I had to do it straight away as part of my trade in so I moved everything over, made sure it all worked and the important apps logged in, sat there for 5 minutes just in case anything came to me and wiped it straight away.
Normally I wait until I send off my old phone for trade in. Last year I just went to the Apple Store as I’ve heard nightmare stories about mailing them in!
I do the direct device transfer when setting up the new phone, and once it's successful I allow it to wipe the old phone and get it ready for trade-in.