Still another slight variation on the theme.
Considering the Vy situation, which is the thrust (pun intended) of the question ... this relates to the maximum available excess power.
As altitude increases for the typical small aircraft, the net available power decreases while the component curves of the power required curve vary to cause a slight skewing of the combined curve.
The common result is that the speed (TAS as we are talking about power) for maximum excess power increases slightly with altitude. The corresponding CAS will vary as normal with respect to the density. The end result is that CAS may reduce slightly ... one would need to consider the situation in relation to the specific aircraft being looked at ....