for the better 几处字典的解释：
If something changes for the better, it improves;
in a way that is an improvement；
to be an improvement.
所以原句
The world has changed for the better. 等于 The world has improved.
翻译为世界变得更好了是很确切的.
而世界有更好的改变,和原句有差异.
The world has changde for the better.可翻译为世界变得更好了,但老师翻译为世界有更好