Inspired by that question: How is the airspeed-Mach number transition handled in modern airliners?
When pressure and density decrease IAS also decreases. When temperature and pressure/density decrease speed of sound also decreases. As air pressure, density and temperature all decrease with altitude, if one fly a constant IAS or a constant Mach number while constantly climbing or descending, the aircraft actually constantly accelerates or slows relative to the air mass.
But at lower cruise altitude an aircraft will be piloted to maintain a constant IAS, while at upper cruise altitude a constant Mach will be flown.
Machmeter combined with airspeed indicator, source
What is the reason for changing the speed reference (IAS or Mach number)?