phtcloud As Spinebot replied, to upgrade, first import the .skel file into Spine 2.1.27, save it as a Spine project, open the project in Spine 3.8, and save it again.
These operations can be performed from the command line. (Spine 2.1.27 is a very old version, so I apologize if it doesn't work properly.)
Please refer to the following page of the user guide to learn how to import skeleton data from the command line and save it:
https://esotericsoftware.com/spine-command-line-interface
