Change the permissions of items in bulk in FoundryVTT.

foundry-item-permission.js

var folderName = "Test";
var reqPermission = 0; // 0=None, 1=Limited, 2=Observer, 3=Owner

/*****
WARNING: This will overwrite the permissions of the items in the folder <folderName>!
         That is, it will not keep the player permissions!
         
         Tested on FoundryVTT 0.5.5
******/

function repermission(currentFolder) {
  console.log("Repermissioning: ", currentFolder.name);

  if (currentFolder.content){
    currentFolder.content.map(item => {
      let newPermissions = duplicate(item.data.permission);
      newPermissions.default = reqPermission;
      console.log("  Item:", item.data.name);
      item.update({permission: newPermissions});
    });
  }
  
  if (currentFolder.children) {
    currentFolder.children.map(({data}) => {
      repermission(game.folders.entities.filter(f => f.data._id == data._id)[0]);
    });
  }
}

// select folder to re-permission
var rootFolder = game.folders.entities.filter(({data}) => data.name == folderName)[0];

repermission(rootFolder);