Ah, I see, people may be depending on rename on Windows not overwriting.

I suppose a new function (and eventually deprecating the old?) would be the most straightforward way forward, though I dislike the necessity :)

An alternative might be a flag on rename: overwrite=['always', 'os_default'], with a warning and a switch of the default in a subsequent release.  That's ugly too, of course.  The existing per-platform variation in behavior of rename gives us a mess to deal with.