The expansion cards are interoperable between the FW13 and FW16 models. The FW16’s 180W power supply is backwards compatible with the FW13 as well. The FW13’s 60W power supply technically could be used for the FW16 as well, although that is not ideal.
Obviously, the RAM, M.2 NVMe SSDs, and M.2 WiFi modules are all interoperable as well.
Although the FW16 uses DDR5 and most of the FW13’s use DDR4, so obviously those wouldn’t be swappable.