Interfaces and APIs is what you give away to others. If you keep changing them, you'll make everyone's life sad.
When talking about boolean parameters, I mentioned about renaming the function. If you control the whole source where the function is used, that's not issue, it's just a matter of search and replace.
But if that function was actually exposed in a library, you shouldn't change function names in a whim. That will break a lot of other applications beyond your control and make a lot of other people unhappy.
Remember, when you write tests for APIs, you can see these kind of changes happening and you can see the kind of changes you're doing on how they reflect externally.
You can create the new functions and mark the current one as deprecated, either by documentation or by some code feature. Then, after a few releases, you can finally kill the original function.
(A dickish move you can do is to create the new functions, mark the current function as deprecated and add a sleep at the start of the function, in a way that people using the old function are forced to update.)