In a recent blog post I explained why APIs used to be completely frozen once they were published, and how we have loosened that restriction for Public APIs. Similarly, we did not allow Client Apps to be changed and then re-registered. This was never a good decision, since the Client App does not have anything "connected" to it (the way that an API may). So we should never have restricted the registration of a Client App!
When Can I Re-Register?
A Client App can be re-registered to the Gateway whenever it has been modified, or if it has been un-registered (is in the Retired state).