For JAR ATPL(A) purposes you need to think of the way reducing air density affects the thrust.
If we ignore any changes in mass and any compressibility effects, then at constant CAS we will need constant angle of attack to give constant lift. Constant angle of attack at constant CAS will give constant(ish) drag.
But as altitude increases, the thrust available will gradually decrease. So to maintain constant CAS we must reduce the drag to match the reducing thrust. To do this we must decrease the angle of attack.
All of the above is typical of the simplfications that are used in JAR ATPL(A) exams.