var arrayOfObject = [{'key1': [1,2]} , {'key2': [1,2,3]} , {'key3': [1,2,4]}];

​

var keyString = 'key1';

var newKey    = 'newKey';

​

var newArray = arrayOfObject.map(function(item) {

  if (keyString in item) {

    var mem = item[keyString];

    delete item[keyString];

    item[newKey] = mem;

  }

  return item;

});

​

console.log(arrayOfObject);

console.log(newArray);