So sorry for the delay getting back to you here!!
In sum, you are right! The $update(owner_guid = ...) function needed to be ported to the newer version of the API. It also requires RStudio Connect 1.8.6. If you update to the newest version of the connectapi package (0.1.0.9018 at the time of writing) and ensure you are running RStudio Connect version 1.8.6 (latest patch release is 1.8.6.2), then this function should work now!
Please let us know if you run into any more trouble!! Hopefully this will fix your issue!