That not correct. Methane is very good even if you only go to the moon. And if you produce fuel on the moon its still perfectly reasonable option.
And also Raptor engines were not a redesign but a completly new design. Even if you stick to RP-1 they would have moved to a close cycle and a new engine.
The main reason for methane is reusability. There is a reason RocketLab, BlueOrigin, SpaceX and Stoke all switched methane for next generation reusible engine.
And no you can't just scale up F9. That complete nonsense. at most you could build a new rocket with the same fuels and materials. But even then, the difficult part is the reusable second stage and that is a 100% redesigned.