avc8130, first off, the vents will work as a cold air return, that is assuming the heat has another way up like a stairwell. Heat seems to work sorta like electricity, it takes the path of least resistance, therefore, it will still go up the stiarwell, but the colder air in the bedrooms will fall through the vents and be replaced by warmer air.
Now, on to the risks of cutting vents.
It may or may not be against local building codes, so check that. Also, there are vents made with a heat activated, automatic closing system. Remember the vents will work as a cold air return so with these vents, god forbid there is a house fire, they close automaticly, but otherwise they allow the cold air to fall. Something you should absolutely consider.