I'm starting to lean a bit more towards a new function and a clean break where the old one doesn't call the new one but I just leave it untouched to die (remove in a much later release). I recently added https://github.com/voxpupuli/puppet-ldapquery/pull/47/commits/47d523c946109556cc5650c0e1eaea3679dc3fbe and whilst I think this is a sensible approach it'll change the behaviour of the existing function/dispatch.