Bebekkun How to roll back a Spine project saved in a newer version of the editor to an older version is explained in this section of the user guide: http://esotericsoftware.com/spine-versioning#Recovering-work-from-a-newer-version
But to begin with, there is no need to roll back the patch version numbers. Updating patch versions is always safe. Please tell your client that you can update to 3.8.99 without worrying that something breaks or that your exports will no longer work with the 3.8 Spine Runtimes.