At some point java.lang.Override started to be available for use with implementations of methods declared in interfaces. I'm pretty sure there was a time when it just worked for overrides of methods defined in superclasses.
How can I find out when (i.e. at which version) this change happened?
I don't know how you'd find this out, but it happened between 5 and 6. (i.e. it's forbidden in 5 but accepted in 6.)
Use of the @Override annotation on methods that are implemented from interfaces and not overridden from a superclass is a new feature in Java 6. See @Override specification changes in Java 6:
Between Java 5 and Java 6 changes to the specification of @Override have been made. In Java 6 it is possible to add the @Override annotation to methods that implement methods of an interface which is not allowed in Java 5.
I noticed the difference when a friend told me that he had to remove all the @Override annotations to make his Java 6 project compile with Java 5.
The interesting thing is that there is no documentation about this change. The API specification of @Override is exactly the same in both Java versions. I found out that this was forgotten by Sun developers. Peter Ahé, a former developer at Sun, calls it the @Override Snafu.
You cannot "officially" find out because someone at Sun messed up and did not update the specification in the API doc of java.lang.Override when the implementation was changed, and apparently changing the specification after the release is not allowed.