That would include writing new drivers for all the components and the SoC is not really the weak point of the FP2. The week point rather seem to be Google’s dumb upgrade restrictions.
If they make a new main board with a different SoC, they can in the same go make a smaller screen and further improve the modular overall design.
In the end, I think it is also a question of customer “confusion”: Which modification is enough to justify a version jump from (Fairphone) 2 to 3? Is it simply a different SoC, while the rest of the phone stays exactly the same? Or do other qualities of the phone also need to be modified to make the change recognizable for customers?
What I don’t like at all is the “alphabet confusion” Samsung creates (links to Wikipedia): Samsung Galaxy A, C, E, J, On, S, Y. In many cases they even have a 2016 and 2017 edition (or 2015/2016). One aspect that shouldn’t be forgotten in this regard is the aftermarket. The many versions and models make the smartphone landscape extremely fragmented and hard to maintain for community developers.