Pace,
Sorry I don't understand. Are you saying that the IAS of Vx decreases with altitude ?
Theoretically, there should be no difference to the Drag curve with gain in altitude. As per the lift equation, for the same IAS, Lift and Drag will decrease in equal measure and must be accounted for by an increase in TAS.
There will, however, be a decrease in propeller thrust due to decreased density and so there will be a decrease in angle of climb but the best angle of climb will be achieved at the same speed.
The Rod Machiato article merely explains an increase in the TAS of Vx which is due to the need to fly faster due to reduced density. But it does not explain an increase in IAS