I never quite succeeded in this, myself. What I do remember is that in 2017, CAESAR added the capacity to have 6 restraints instead of 4. That would have to be something that's corrected if applicable.

Then it's a matter of exporting to a neutral file, which is text-based, and modify it to fit what 2013 expects a neutral file to look like.

Unfortunately, I don't know what all steps that might include, but you might be able to make smaller test files and compare those to get some idea.