In general, input files are not "downward compatible", this is clearly stated in the Distribution Letter that accompanies each release. The reason for this is that newer versions typcially contain new data fields that earlier versions did not. So attempting to open a newer input file in an older version would lead to a crash.
If you use the [Search] feature (to the right above the calendar), and search for "compatible" you will find a number of posts discussing this subject.
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Regards,
Richard Ay - Consultant