Fastcruise, this is a very common problem, even when in VNAV, it can be avoided to an extent by putting in the descent winds and hence a more accurate TOD, if speed is 10kts above FMC speed, it will revert VNAV Speed VNAV PATH. LVL change is essentially the mode VNAV will use as it assumes thrust is idle at TOD. If you get the msg " Drag required" use the speedbrakes or you will get " overpseed disconnect" and CWS reversion. One technique I find useful is to allow the a/c to start the descent in VNAV or level change and when the speed stabilises select VS to maitain the ROD, allowing for the pitch reduction in the last 1500ft as preferred by ATC. As mentioned, you can easily go through VMO using VS, which is why it is better for small changes when momentum cannot build up. Of course if you use LVL change above Mach transition, remember to manually revert to IAS below Mach change over or you will rip the wings off!