The classic (well, dunno about the 400, but 500 and 300 work that way) speed control switches form IAS to Mach at FL260 if in level change or V/S.
In VNAV climb it will keep the IAS in the mode selected (ECON most likely) until the corresponding mach number of that profile is reached and then keeps that. That way switching usually is done later, often around the FL280 to FL310 mark depending on cost index and entered top of climb wind.
During VNAV descent its just the other way round, the FMC keeps the mach number according to the selected cost index and switches to IAS when that is reached. Descent speed schedule is not variable but rather fixed speeds according to the selected CI. With a low CI it often happens that the switch to IAS is done immediately upon selecting VNAV descent, for example we use a fixed CI of 20 which corresponds to 266 kias descent speed and that is often enough reached at our normal cruising levels, since 266kias isn't really ATC friendy most of us change the descent speed on the descent page to 280kias which is reached a tad later.