PilotDAR,
The 100RPM & 10kts increase is just an example.
The situation is cruising at the same altitude over long distance, I enter an area of significant atmospheric condition, maybe an area where the temperature drops significantly & density increases due to whatever phenomenon (cold front or something like that, just as an example).
That would cause the engine RPM & thus airspeed to change.
Now the leaning was done earlier, before this atmospheric condition change occured.
The question is, other than adjusting the throttle to maintain the original intended airspeed (reduce the RPM to lose the extra 10kts gained), shouldn't the mixture be readjusted again due to this "new" atmospheric condition?