Update -
changing the Endpoint in the TestRequestProperties is the way to get the POST to change.
However, this can be a little quirky.
Also, the XML in the request may or may not change when the "change operation" is used -
I haven't been able to find a fixed set of steps to repeat the problem, or to clear it.
What I've found is that I can eventually make the new test step work, by a combination of saving the project, editing the request's XML in the XML tab, closing the test step interface and editing the Testrequest properties.
I think the key is closing the test step's interface - I think this is acting like an editor that is cached and not completely refreshed when the underlying test request has changed.
