title: Changing source branch for PR on Github
body: <p>I accidentally created a pull request from the master-branch of my fork of a repo.</p>
  
  <p>While trying to rebase it I noticed that all of these changes were pushed into that pull request — due to the fact that you can simply <code>Add more commits by pushing to the master branch on username/repo</code></p>
  
  <ul>
  <li><strong>Can you change the source branch of a pull request <em>after</em> the pull request has been submitted?</strong></li>
  </ul>
  
  <p>I see you can edit the base branch, but that's obviously not what I'm after.</p>