title: git subtree: possible to change subtree branch/path in a forked repository?
body: <p>In a repository <code>A</code> the folder <code>sub</code> is included as git subtree of the repository <code>S</code> - pointing to <code>master</code> branch.</p>
  
  <p>I have forked repository <code>A</code> into <code>F</code>. Now I want to do one of the following in <code>F</code>:</p>
  
  <ul>
  <li>change <code>sub</code> to use a different branch of <code>S</code> (ie <code>develop</code> branch)</li>
  <li>or: change <code>sub</code> to use a different repository altogether</li>
  </ul>
  
  <p>Is either one of these possible, and if so, how? Will there be any side effects I should know of?</p>
  
  <p>And how can I make sure my subtree change won't be updated in repository <code>A</code> when I merge my changes (pull request)? I mean besides isolating commits.</p>