Can I upgrade the logic board

My research says no.

So far, everything points to Apple making sure that there are little, but crucial, differences between the boards, even in the same series, to make sure that they can't be "upgraded". Even if the boards fit, there are likely connector differences, a screw hole placed differently, etc.

Even with standard non-Mac laptops, the boards are proprietary and are generally not compatible. The only exception is sometimes, companies like Dell use a particular board in more than one line, though that only means you can use a replacement board from another system. It doesn't address the "upgrading" aspect.

Apple makes a solid product, but their whole business model is based on selling a whole, complete product. "Upgrading" is a PC thing...something Apple tries very hard to contradict in their designs.

While I hope this answers your question, on the other hand, I kinda hope someone can prove me wrong. :p