Please check the save_system command:
E.g. I find "filename = save_system(sys,newsys) saves the model to a new file newsys. If you do not specify an extension, then save_system uses the file format specified in your Simulink® preferences.". So I would expect if you set that in Simulink's "Model File Preferences", it would work. Try it out.
I wonder however why you want to do this. I have a certain understanding in users have sometimes a need to backport to an older release, but why using the older file format in the same release?