Going back to NW1's original point - it isn't a problem in a modern flt system to transition from a Mach No to an IAS.
The pilot can put the 2 values into the FMS as a Tactical mode, or can use the flt system. For example, on the F100 selecting Mach Hold on the glareshield then pre-setting the desired IAS will ensure that the flt system switches from one to the other, and maintains, at the appropriate point.
I think the Bus works in a similar manner.