Is it possible to change the name on an existing terminal rule?
For example, there is a terminal rule defined for "hostnameA" but it needs to be changed to "hostnameA.domainB.com".
No, there is no way to change the "terminal name" of an existing terminal rule.
The closest option is to add a new separate rule for the new terminal name, then refer to the info from the old one to duplicate the settings and rules that have been applied to it. Once the new rule has been created the old one can be deleted.
Documentation on managing TERMINAL rules: