What it Took!
I've always had good luck with the phone-to-phone transfer when upgrading, but just to be safe I always make a complete backup of the old phone to my laptop just beforehand. Haven't needed it yet, but it's nice to know it's available.
It’s worth spelling out the roadblocks that blew up my phone-to-phone transfer.
Started out well, got the eSIM info transferred right away, passcode credentials, set up Face ID, iPhone updated to 17.4 which took a while, then the new iPhone got stuck in some AppleID sign-in mode for over 30 minutes. Even though it had already successfully signed in over 30 mins ago as I got a new device signed in notification email from Apple. Maybe it had to resign-in after the updating it’s OS to the latest and that’s where it would get stuck.
Cursory web search it immediately pops up that lots of people had their iPhone 15 phone-to-phone transfers stuck at this point. A long Reddit thread spelled out details. Crazily most people were successful getting past this point by changing their Apple ID password.
So after trying a few things but always getting stuck at this dubious sign-in step I decided to change the Apple ID. Well the AppleID website wouldn’t let me do that because I had “Stolen Device Protection” on on my iPhone. It gave me a list of my devices where I could change the AppleID password but not how to do it without using the web interface. So we ended up turning off the protection on the old iPhone since the new phone was still stuck somewhere in setup where you couldn’t get to anything. I figured out afterwards how to change the AppleID password from System Preferences iCloud options on a laptop or on a iOS device Settings top level top option with your name. Good to know. These things are impossible to search for under Settings anymore. Drives me crazy.
AppleID password changed worked! After forcing restart again it immediately asked for the new password and no longer got stuck in the signing in step. Finally! Now after a couple more steps we got to the all important data transfer stage. I keep a lot of data on my iPhone mostly images and videos so this step can take a while. Well much to my chagrin after about 30 minutes big “Data Transfer Failed” banners on both phones. Looked like transfer got about 80 complete before failing. So I reinitiated this transfer step to try again. Exactly same failure mode the same point.
So I gave up on phone-to-phone transfer. Fortunately I had backed up my old iPhone 13 on my laptop earlier in the day (older OS via iTunes) and initiated a restore from that. Thank goodness this worked seamlessly - just another 30 minutes. And after that succeeded was prompted to connect my existing Apple Watch which worked.
Another thing worth mentioning is that after some setup step failed (instead of just getting hung up) is that the new iPhone would give you the option to reset to factory settings. Well the eSIM credentials had already been transferred to the new phone and wiped on the old, and I didn’t want to chance losing that. So I did the force power off method instead (power button plus volume up button held down). This would generally let me get to a prior setup step and continue from there. We learned later that starting in iOS 15 the factory reset gives you an option to preserve the eSIM - good to know. However the force power off method I used did let me get back to a previous step during setup which meant I didn’t have to keep redoing everything else. I was able to get to the step that let me pick restoring data from my laptop instead of the stage with the phone-to-phone transfer.
DH and I both worked on this for hours which included several web searches which DH undertook after my initial problem. Worth mentioning that we both retired from electrical/computer careers with a lot of software experience as well, and have owned Apple computers and devices forever so we are pretty tech savvy. I guess many folks can run to their Apple Store but our closest is 250 miles away.