CAESAR II neutral file converter is not forward compatible. But I'm sure you can find a way to tweak the 2018 neutral file into something that CAESAR 2017 can interpret. But you will lose some data since the newer version has new features not found in the older version. And depending on how good you are with computers, it may end up being more time consuming that remodeling everything in 2017.

I suggest that you read the neutral file format description in Section 15 of the User's Guide of both version 2017 and 2018 and get familiar with the formats. There's no quick answer to this.