I think you misunderstood (at least, I hope I've understood :)

Rewriting rename to include the -i option (or -y option for Windoze) will not clobber an existing file (unless the underlying OS function has the same problem with the -i option, in which case we're all screwed.)

I tried my example out, and it seemed to work as I intended. I'm sure someone has come up with a more robust and useful version. I was just trying to show that it was possible.