I upgraded unity-spine to 3.8, my existing Spine .JSON files (3.6) were not loading anymore inside Unity and the artist who made them was not available to re-export them (20-30 files).

To fix that, I made a small CLI PHP script that work good enough for me, It's a quick 30 minutes hack but It could work for you too, here is the code, for me it was a huge time saver:

<?php

set_time_limit(0);

function convert_file($file_input) {
   

   if (!file_exists("input/".$file_input)) {
         die("File not found!\r\n");
   }

   $data = json_decode(file_get_contents("input/".$file_input));

   $data->skeleton->spine = "3.8.91";

   $skins = $data->skins;
   $new_skins = [];

   // Fix to skins

   foreach($skins as $key => $value) {
      

  $item = [];
  $item["name"] = $key;
  $item["attachments"] = [];
  
  foreach($value as $k => $v) {
     
     $item["attachments"][$k] = $v;
  }
  
  $new_skins[] = $item;
   }

   $data->skins = $new_skins;

   $animations = $data->animations;
   foreach($animations as $key => &$value) {

  foreach($value as $k => &$v) {
     foreach($v as $k2 => &$v2) {
        
        foreach($v2 as $k3 => &$v3) {
           
           if (!is_array($v3)) continue;
                 
           for($i=0;$i<count($v3);$i++) {
              if (is_array($v3[$i])) continue;
              if (isset($v3[$i]->curve)) {
                 $c = $v3[$i]->curve;
                 $v3[$i]->curve = $c[0];
                 $v3[$i]->c2 = $c[1];
                 $v3[$i]->c3 = $c[2];
                 $v3[$i]->c4 = $c[3];
              }
           }
        }
     }
  }

   }

   $data->animations = $animations;

   $fd = fopen("output/".$file_input, "w");
   fwrite($fd, json_encode($data,  JSON_PRETTY_PRINT));
   fclose($fd);
}


$dir = opendir("input");
while($file = readdir($dir)) {
   if ($file[0] == ".") continue;
   if (!is_file("input/".$file)) continue;
   if (strpos($file,".json") === false) continue;
   

   print "Processing file: ".$file." ...\r\n";
   convert_file($file);
   

}

print("Done.\r\n");

To run it, you need to create 2 sub-directories ("input" and "output"), then when the script runs he convert all JSON files inside the "input" directory and generate the fixed JSON files in "output" directory.