Computer

They would need to have the same drivetrain options - differential gearing, tranny gearing, same size engine, same sensors, and here is where things get funky, year to year changes may make one years computer incompatible with another years engine, especially with transmissions which change the most (torque ratings, shift points, etc). You can only re-flash an ECM to a factory setting so you can't really create a "custom" to match a hodge-podge of parts, unless you go to an aftermarket chip. But overall you can generally swap ECMs and flash as needed but be sure to consult a mechanic or whoever you intend to use to reflash it on compatibility issues. (no matter what they have to flashed to match vin, mileage, anti-theft)