I think it is not in a fix point neither an altitude. In your fms you will see the speeds your AP will maintain during climb and descent. During climb it will maintain IAS. The Mach number increases with the altitude maintaining the same IAS. When required mach number is reached then it will maintain that Mach Number and the MCP changes to Mach. On the descent works opposite. It will maintain the Mach until it matches with the required IAS. So the point of change depends on the speeds calculated by the FMS and the point both matches which is related to TAT (True not total).
Imagine it was a fixed level. Depending on the outside temperature you could exceed the Max IAS or Mach Number.
Nice flights.