Summary:
Changing an existing stage is an important feature available in the editor, but none of our existing tests cover this path. A simple ATH test (or even an addition to another, existing test) would be a nice coverage improvement.

Ideas:
An offline test could create a repository including an existing Jenkinsfile, stored in our resources directory, like we're doing here. This existing Jenkinsfile might include something like:

stage ("do-one-thing) {
    echo "Do one thing"
}

which we could then use the editor to change into

stage ("do-two-things) {
    echo "Do two things"
    sh "netstat -a"
}

And then verify that we went from doing just one thing, to doing two things. The same test could delete a step as well:

stage ("back-to-one-thing) {
    echo "back to doing only one thing"
}

In this way we'd cover stage renames, step additions, and step deletions, all in one case.