title: Can existing virtualenv be upgraded gracefully?
body: <p>I have a virtualenv created for Python 2.5 and want to "upgrade" it to Python 2.6. </p>
  
  <p>Here is how it was originally set up: </p>
  
  <pre><code>virtualenv --no-site-packages -p python2.5 myenv
  </code></pre>
  
  <p>I now run virtualenv in the same directory to upgrade: </p>
  
  <pre><code>virtualenv --no-site-packages -p python2.6 myenv
  ...
  Not overwriting existing python script myenv/bin/python (you must use myenv/bin/python2.6)
  ...
  Overwriting myenv/bin/activate with new content
  </code></pre>
  
  <p>The default python is still 2.5, even though I can also specify 2.6.  Is there any way to <em>remove</em> 2.5 entirely and have 'bin/python' point to 2.6 instead? </p>