I'm not sure it would be sensible to do this. The session file contains data derived from the original file (eg to create the overview window).
If anything significant had changed in the original file (eg if the length is not exactly the same), then it would probably crash the program. It wouldn't be difficult to put in the option to open an alternative file, at the user's risk. If you think this would be useful, I can include it in the beta and see what comments it attracts.