You want to avoid going back to an older version whenever possible because data is likely to be lost. You may need to fix up your skeleton and animations afterward. Is there any way to avoid needing to go back? If not, maybe you can avoid needing to go back so far? 3.5 is very old.
JsonRollback in the Skeleton Viewer supports going from version 3.3.xx to 2.1.27. I'm guessing that's why your 3.6 -> 2.1.27 conversion didn't work out.
I'm not sure what the difference was between 3.5 and 3.6. Actually I used GitHub to compare the 3.5 and 3.6 branches and there doesn't appear to be any changes to SkeletonJson (which loads the JSON). Have you converting to 3.6 JSON and loading that with 3.5?
