I'm one of those rather peculiar programmers who would finds excuses to write library functions 8-), so I can understand why you write a module.

I tend to like your 3d option (complete re-write, keep the old interface for the existing code, and make the new interface available for new code) the most attactive from a technical point of view, and your second option most from an overall point of view. I really do not like either of the last two options, which (in my opinion) are really different ways of doing the same thing.

Added "attractive" so the sentence actually makes sense.