Switchover between 200 and 250? That is mighty low. We do standard switchover at 295 and FMC switchover is usually higher up to FL360.
From PSR's post he has a grasp of the Mach/IAS concept and of switchovers. The instruction seems quite clear to me, fly whatever mach number you want until you reach an IAS of 270kts and then fly that IAS. Not that hard to grasp and even easier to program into an FMC. Speed control is there to squeeze more of us into a given constant airspace and of course prevents slot restrictions and holdings, it is in our best interest to follow it as we follow every other instruction by ATC. And if we can't "unable" seems to work quite well.