"API design is like sex: make one mistake and support it for the rest of your life" (Josh Bloch on twitter)
There are many design mistakes in the Java library. Stack extends Vector (discussion), and we can't fix that without causing breakage. We can try to deprecate Integer.getInteger (discussion), but it's probably going to stay around forever.
Nonetheless, certain kinds of retrofitting can be done without causing breakage.
Effective Java 2nd Edition, Item 18: Prefer interfaces to abstract classes: Existing classes can be easily retrofitted to implement a new interface".
Examples: String implements CharSequence, Vector implements List, etc.
Effective Java 2nd Edition, Item 42: Use varargs judiciously: You can retrofit an existing method that takes an array as its final parameter to take varags instead with no effect on existing clients.
An (in)famous example is Arrays.asList, which caused confusions (discussion), but not breakage.
This question is about a different kind of retrofitting:
Can you retrofit a void method to return something without breaking existing code?
My initial hunch points to yes, because: