There are two ways to get into trouble (at least!) One is to fly at too high an IAS and tear the wings of and the other is to fly at to high a Mach number, lose control and tear the wings off.

For an average transonic passenger jet at low level you will hit the IAS limit, Vmo, first. As you go higher, for that same Vmo both TAS and Mach number increase. Eventually, at this same Vmo you will hit Mmo. As you go higher you will reduce IAS below Vmo to hold Mmo.

The crossover point depends only on the values of Vmo and Mmo. Low Vmo and high Mmo and the crossover will be at high altitude, and vice versa. Typical values are illustrated on the buffet margin diagrams in P of F. So, for the Question, you start your descent at Mmo, watching the IAS rise, and when it hits Vmo that becomes your new limit