04 to 02 engine swap

Are they the same engine, same tranny, and same axle code in both cars? If so you can swap the motors and likely keep the original engine control module (so that you won't have to fuss with reprogramming for anti-theft). I'd also swap the motor and tranny as a pair. I'd look at joining an Impala forum and ask what differences may exist between the two engines (assuming both have the same), otherwise the ECM may need to be reflashed. It's nice to have access to the donor car during the swap, so you can grab any part of a harness that may not be matching up.