At least the last two have required updating, who knows what version of iOS 26 was available when they were packaged. I think it's been more years, but think upgrading is now required before Migration. You would probably not want to use the shipped iPhone's version this week, especially as it would be older than the version on your current phone.
Also, as iOS improves through time, the migration process also is very likely better with both phones on iOS 26 rather than migrating from iOS 18.
I'd love to see a scientific comparison of the migration methods. I have used iCloud backup restore, and tried phone to phone, and also encrypted backup restore from my Mac. As I mentioned above, iOS 26 likely has some optimization that would make one of them "the best" this year. With almost a TB of stuff on my current iPhone, I was thinking of using a Thunderbolt 5 cable between phones, or between encrypted backup on my Mac to the iPhone. I think iCloud restore required signing back into too many apps, though that is anecdotal and may not be completely accurate, part of why I'd like to see a scientific assessment.