Welcome to the Fairphone community forum! 
Basically, the answer is quite straightforward: no. The different components can’t be changed independantly. Only the modules can theoritically receive an upgrade, as for the camera modules.
As to whether there could be a new motherboard for FP3, the answer is no as well. It is very complicated to change the motherboard of a phone since it is equivalent to creating a new phone compatible with the modules of the old ones, and the core-module is responsible for 75% impact of the phone, which means the point is keeping it as long as possible.
In both cases, it would mean developing a new OS for each different combination possible, since such components are very intricate and depend on each other to work, you can’t just change one part and think everything will still work the same, it would require new drivers for each new module, i.e it would be nearly impossible. But let’s dream! 
Fairphone has gone further than all big players, let’s see how far they can manage to go!
This has already been discussed on the forum btw.:
Edit: Ok, obiously I took too long to write my post