WILL A 2002 4.2 TRAILBLAZER ENGINE WORK IN A 2008 TRAILBLAZER
02 is like a unicorn as it's the first year of production. I'd stick with 06-09.
Adaptation isn't what it used to be. With today's computers and sensors, if you have the same model year and the same basic platform, the engines would have been manufactured the same. Replacing an engine with one from a different year or a different platform would entail some modifications, but as the engine is newer than the car it should be easier, as the engine is more backward compatible. But you're proposing the opposite, and that likely wouldn't work without computer modifications.