When replacing a pump in a well the well pump should not be set at the very bottom of the well to prevent the pump from picking up sand etc.
It's recommended that you place the well pump from between 10 to 20 feet above the bottom of the well if possible.
However if you cannot place it that high then don't place the well pump lower than 5 feet from the bottom of your water well.
10 feet minimum from the bottom of the well though is recommended if possible but any lower than that you risk picking up sand etc from the bottom of the well which could destroy your well pump.