My AT&T line was primary, then after a restore, it became secondary, but no real problem.The whole process has become so easy. I actually transferred my AT&T automatically, I have another line with USMobile (my old number) that I keep for stragglers that haven't updated contacts. Anyhow, I did that one manually and it could not have been easier. Website, transfer line, new QR comes and away you go.
Why didn't I transfer them both automatically? Well.. on my 16PM my AT&T was line 2 and USM was line #1... the OCD wouldn't let me do that again when AT&T is my primary.
Until Apple wanted me to designate/assign Verizon as my main carrier when I purchased it.
With iOS 26 on my new 17 Pro, it became #1 again.