You can still run Spine Editor version 2.1.27, import your .json file, then save the .spine project. Next, start a newer Spine Editor version an open the .spine project you just saved. Fix up a few things (flip x/y doesn't exist anymore for example), save, and export a new .json file. That is a much less error prone workflow than trying to convert the old .json files to new .json files, which is usually not possible automatically.