Whilst not 100% on this idea, it might be reasonable include a menu option.
Perhaps the options here aren’t quite correct - but hopefully this shows the idea.
You could also perhaps slip “Official Warning” and “Whisper” in there as options too.
This would also open up a place to indicate a “call” type as required by Start a new conversation with a customer not yet in Discourse feature request.
The idea is to change the type of reply whilst editing.
In the same way as now clicking the reply button on a topic / post would continue to change the type / or update the “linked topic”.