I just wanted to make sure that the newest chilkat.jar (9.4.0 jdk6) is backward compatible with the 9.1.2 version that we have.
The release notes for each version are listed here:
(Backward-compatibility issues are identified.)
In general, it should be backward compatible. However, you should never just drop in a new Chilkat version without testing prior to using it in production. If you find a compilation error, then it should be easily fixed by writing the method call to fit whatever has changed (such as changing the method name, order of args, or omitting/adding an arg). (There have been very minor changes to ensure that the API's across all languages are identical and consistent.)
Finally, bug fixes by definition break backward-compatibility. It could be that your app "worked" because of the behavior of a certain bug, and the bug fix actually adversely affects your app. (Bug fixes are listed in the release notes.)