Open your old PT5 session, select unused > delete, select all, hit compact, hit OK. Pro Tools will compact all your audio files and hang in the end as it will be unable to save the session as it was originally saved in older format session file and all you can do is force quit PT. Now your old session file will not recognize your audio files anymore and you are pretty much screwed.
Only way to fail this otherwise easily successful destruction of your session is to immediately save the session file in new format after opening it up, would be nice if PT would suggest it first thing after opening old session file!!