They did a video where they swapped parts from a first generation FW13 with a FW13Pro. There were only two caveats: one, if you want the Pro battery on your older laptop, you have to buy a new bottom panel too because they made the battery bigger for a longer life and it won't fit in the old cover. And two, depending on your architecture, the Wi-Fi modules might not be interchangeable due to some vendor chipset thing, I think?

Everything else was able to be swapped, though, it seemed.

Edit: oh wait, I take it back, the speaker modules. I think they might technically be compatible, but since they're side-firing on the Pro instead of down-firing, you'd need a new bottom chassis if you didn't want them to sound awful.