What we learned from the teardown is that the iPhone SE actually has many interchangeable parts from the iPhone 5S. The most important being the display, which is identical to the 5S and can be swapped out using the same parts for that model.
Other interchangeable components include the speaker, chassis, vibrator, SIM tray, display assembly, digitizer, front camera, earpiece speaker, and proximity sensor. Unfortunately the parts that are not interchangeable include the logic board, rear camera, Lightning connector, and the battery.
The iPhone SE's camera and battery both have new connectors, which cannot be used on an iPhone 5S. Folks who were hoping to swap out their 5S battery with the larger one in the SE are out of luck.