have changed or have been changed

[Sung_Tung]

Please help me with this sentence. I don't know it should be 'have changed' or 'have been changed'
Theories about the causes of bird flu have changed/ have been changed in the light of recent research.

[Peterkua]

Both are correct usage :)

Peter Kua

[Sung_Tung]

Thanks a lot for your feedback but what you think is the better?
Can you give me a example showing that both are correct usage?

[Peterkua]

My flight to Australia has changed from Monday to Wednesday.
My flight to Australia has been changed from Monday to Wednesday.

The above sentences have the same meaning.

[Cerberus™]

"Have changed" simply implies that they changed (intransitive verb), with no reference to the cause of this change; "have been changed" implies that someone or something changed them (transitive verb). In this example, both are possible: "theories have changed" simply means that the theories are different from before; "theories have been changed" implies that someone or something undertook to change them according to a certain purpose.
To me it sounds more idiomatic to assume that theories change by themselves; when you have a theory about something and you receive new information on it, the theory in your mind changes by itself, you do not actually "do" the change - it is an inevitable process. Flights, on the other hand, can go with either voice.