What needs to be done for a 1999 3.4L to go where a 2002 3.4L was because the plugs some of the plugs are different

You will need to swap all of the sensors from your motor to the new motor where the plugs do not match. You will also need to swap the fuel injectors from your motor to the new motor as they are not compatible. The motors themselves did not change between those years.

Check the ECM, I believe they are not the same for those years. It will probably work, but not as well as it could with the correct ECM.

@ Norm the PCM of the '99 is different but is of no concern. The 3400's from both years are pretty much the same engine. As long as the original electrical equipment on the engine is used from the '02 on the older engine it will run normally. In 2000 GM switched between high and low impedance fuel injectors which is why they need to be changed (just remove the upper intake manifold and depressure the fuel rail before starting the swap). (FYI the 3100 was updated around 2000 but even them could be swapped and ran with either PCM with a little more work!) One of the simplest ways to check for sensor/part compatibility is to use an online parts look up database for the two different years of the vehicle. If the same part number is available for both engines, leave it (test it maybe), if they're different, swap it. Temp and pressure sensors are usually good but the TPS, IAC and ICM usually vary. The ICM reads and converts the crank position sensor and controls spark (with a advance/retard signal from the PCM). These vary between years and models of the car based on emissions, power and vehicle (car/mini-van). They can be inter-changed but it is easier/safer to stick with the stock unit that came with the car. Most of the position pick-ups Crank/cam are common units and the reluctor is varied on the shaft it is reading.