He said he's changing it out because the engine's about to go out... so how do you know that he just doesn't want a D-series with lower miles just to keep the car running with the turbo for cheap?

and yes, if he gets another D-series it is still changing the engine. Under your logic, then you're saying if someone swaps a D15B8 for a D16Y8 it's not changing the engine. Those are certainly two different engines, and he would be changing the engine. Also, he doesn't even have to get a different engine code engine to "change" the engine. If he put another engine of the same type in his car, it's still changing the engine because it's not the same EXACT engine.