as 2daddies says, the problem isn't when you are cruising at one level and assigned another (happens all the time ... "when ready descend to FL210, contact centre on 123.45"), it's when you are already on descent. On frequency change, do you report that you have left FL330 when you are now through FL295? If you are now through FL295 do you say "Left FL300"? Or is it "ML Centre, XYZ left FL 295, FL294, FL293 assignedflightlevel210"?