At what altitude does the Mach meter come alive and you fly Mach number instead of IAS and you monitor MMO rather than VMO?
Whats is it that causes the cross over?
The altitude at which the mach meter comes alive depends on the mach meter.
The altitude at which mach becomes a reference in a climb is the altitude at which that mach is equal to the chosen IAS.
For example, if you climb at Vmo at low level the mach number will be low, however, as one climbs maintaining a constant airspeed the mach number will increase (even if you can't see it on the mach meter yet) until you find that at Vmo you are also at Mmo.
If you continue to climb at Vmo then you will exceed Mmo so from here up you use mach as the reference to avoid exceeding Mmo.
If you climb at a lower speed, you will find that there will usually be a certain mach that is maintained once that mach is reached. Again climbing at a constant IAS causes the mach to increase until the chosen Mach is reached and thereafter this mach is maintained.
What is it that causes the change over?
The answer is that climbing at a constant ias causes the mach number to increase and eventually the chosen ias will be more than the desired mach number.
Similarly when descending if one descends at a constant mach number the ias will gradually increase until (if you are not watching it) Vmo will be exceeded.
Therefore while mach will initially be the reference in the descent, eventually the desired ias for descent will be reached and from then on this IAS will be maintained.
In simple terms, it is easier to fly at a constant IAS or a constant Mach and the crossover is simply where constant IAS changes to constant mach (or the opposite).