Create a new namespace for your new API, or your new Interface, or your new version, or whatever you want to call it. But don't delete your old namespace. If your code is someone else's dependency, you should leave the old namespace in place for those who need it.

Left unresolved is how to deal with the accumulation of old code. I assume there will eventually be automated ways of turning it into a library, so the old code no longer needs to appear in the code that you are actually working on. Some automated solution seems like the kind of thing that the Clojure crowd will come up. I think it would only take me a day to write a macro to recreate namespaces from a library. Zach Tellman has some code that does half of this (Potemkin).