As nouns, is there any difference between 变化 and 改变?
a. 城市已经发生了很大的变化。
b. 城市已经发生了很大的改变。

Comparing 变化 with 改变 in this context, 改变 connotes 城市 is being changed (say by people) from the past. Well, 变化 connotes that we see or identify some difference from what the city used to be. 改变 describes the action, change.