A light aircraft will respond quickly to adapt to changes in the velocity of the air mass (through wind changes or maneuvering in a constant air mass)
Manoeuvring in a constant air mass isn't a change in the velocity of the air mass.
However, on the other hand, the kinetic energy of an aircraft flying at 100 knots IAS into wind will be lower than the kinetic energy of the same aircraft flying downwind at 100 knots, in RELATION to the EARTH.
The increase in kinetic energy required to change from flying into wind to flying downwind takes time and is dependent on the inertial mass and drag of the aircraft.
Sorry, you're confusing your frames of reference. Otherwise it'd be incredibly difficult to reverse direction when walking down the aisle of a moving train.
But there have been so many examples in the last few pages showing why the downwind turn thing is a myth - all completely ignored by the true believers - there's probably no point bringing up any more.