The verbs "change" and "improve" are tricky because they could be translated as either 바꾸다/개선하다 or 바뀌다/개선되다. The problem is, when students see the latter, they automatically translate it using a passive voice construction. This is not necessarily incorrect, but could make your sentence stilted and wordy. Unless you really want to emphasize that there is a third-party do-er, I recommend using the active voice.
For example, for 상황이 개선되었다:
- The situation has improved: perfectly fine
- The situation has been improved: this is ok, but makes the reader wonder "by whom?"
- His opinion has changed: perfectly fine
- His opinion has been changed: makes the reader wonder who made him change his opinion
