I just upgraded my version of Eclipse on Ubuntu, and now whenever I open a Python file it opens in an external window, and the existing Python files I have open in Eclipse say "Could not open the editor: No editor descriptor for id org.python.pydev.editor.PythonEditor".
I tried uninstalling and reinstalling both Eclipse and the PyDev plugin, and it didn't fix the problem.
From my experience (~8 years) with eclipse don't do in place updates. Always install a fresh version with a new workspace. And re-checkout projects from SCM.
Check first whether all required plugins still work before switching to the new environment.