I don't want to add the new method to the old interface, because this would mean that an implementation of the interface is forced to implement two methods: the new one, which is probably desired, and the old one, which is just adding code.
The only option we could discuss as an alternative to adding a new interface is to modify the signature of the existing interface. This would introduce an incompatible change, though, and would require use to increase the major version number of the exported package. Not a big problem, probably, but we should be aware. In fact this would enable use really clean up and drop support for the "old" API altogether.