NI Multisim is not backwards compatible, but it is usually possible to import older designs to newer versions of the tool, which is also the case of ms8 to mp9 conversion actually. But this is more of import of designs to make a new project in some very old versions of the software and the latest now is 22. So if you need version 9 files, you will have to obtain that particular version of the software.
Updated: August 7, 2023