title: Update a Python virtualenv?
body: <p>It would be really convenient to be able to update a virtual environment produced with <code>virtualenv --system-site-packages /path/to/myvirtenv</code> taking into account changes that have been done in the base Python.</p>
  
  <p>For example, if IPython 2.4 has been installed in the virtualenv and IPython 3.0 has later been installed in the base Python, the old IPython should be removed from the virtual environment and replaced by a copy of the newer package.</p>
  
  <p>How can we do this? Is there already a command to do this? If not, would it be possible to implement it in the package virtualenv?</p>
  
  <p>PS: Working with virtual environments produced with the --system-site-packages option is very convenient when you do not have admin privileges but if any modifications to the base Python make the virtual env buggy, it does not seem to be a good method!</p>
  
  <p>PS: This issue is related to this question <a href="https://stackoverflow.com/questions/25170016/virtualenv-not-finding-updated-module">virtualenv not finding updated module</a>.</p>