I don't understand the question. The same KTAS as what?
Mach varies with temperature so it's not the altitude that matters, it is the temperature. KTAS varies with temperature and pressure.
As you climb at a constant indicated airspeed, your true airspeed and mach number increases. At some point you reach either the desired, or the limiting, Mach number and transition to Mach.
As you descend at a constant mach, indicated airspeed increases and true airspeed decreases. At some point you either your desired, or the limiting, indicated airspeed and transition to airspeed.
Those transitions normally occur in the FL280 to FL330 range, depending on the conditions and airspeed/mach desired.