Actually the neutral file should be forward compatible. Most of the time the neutral file is not backward compatible - because we add more information to the input that older versions can't read and interpret. Trying to go from 2018 (v.10) to 2017 (v.9) is an example where this won't work. In v.10 we added tags (text strings) to elements, restraints, and hangers (in addition to a few other items). This is why you can't take a v.10 neutral file back to an earlier version.