Go through specific library and set all subfolders as well as the root folders content type using powershell - Stuck

Question

What I am trying to do is access a SharePoint 2010 document library, change all root and sub folders of that library from their existing content type to a new content type. So far my code has somewhat worked but I cannot seem to see the content types although I can get the folders.

    Function RunThroughSubFolders($folder)
    {
       foreach($subfolder in $folder.SubFolders)
           {
            #write-host $subfolder.Name

            $OldCTName = "Document Set"
            $oldCT = $subfolder.ContentTypes[$OldCTName]

               if ($subfolder.ContentType.Name -eq $oldCT.Name)
                   {

                       write-host "OLD CONTENT TYPE NAME FOUND!("$oldCT.Name ")" $subfolder.Name

                       #write-host $subfolder["ContentTypeId"]
                        <#

                         $subfolder["ContentTypeId"] = $newCTID
                         $subfolder.Update()

                         #>

                    }
                    else
                    {
                     ##
                    }
             }
     }

     Function GetFolders($Folder)
       {
          foreach($subfolder in $Folder.SubFolders)
              {
                 RunThroughSubFolders($subfolder)
              }
       }

     $web = Get-SPWeb "//url//”

         foreach($list in $web.Lists)
           {
             if($list.Title -eq "Document Library")
                {
                  GetFolders($list.RootFolder)
                }
           }

Answer 1

ContentTypes and ContentType are not properties of an SPFolder object.

So your line

$oldCT = $subfolder.ContentTypes[$OldCTName]

will not work, and neither will

$subfolder.ContentType.Name -eq $oldCT.Name
#...
$subfolder["ContentTypeId"] = $newCTID

To fix the first part, I would recommend getting the SPContentType object from the list, so maybe something like:

if($list.Title -eq "Document Library")
{
    $OldCTName = "Document Set"
    $oldCT = $list.ContentTypes[$OldCTName]

    # multiple parameters are passed in Powershell as space separated, not comma separated
    GetFolders $list.RootFolder $oldCT 
}

and then pass the content type object on to your next function(s) in a similar way

Function RunThroughSubFolders($folder, $oldCT)
{
    # ...
}

Function GetFolders($Folder, $cType)
{
    foreach($subfolder in $Folder.SubFolders)
    {
        RunThroughSubFolders $subfolder $cType
    }
}

As for the second problem, what you probably want to be doing is

if ($subfolder.Item.ContentType.Name -eq $oldCT.Name)
{
    $subfolder.Item["ContentTypeId"] = $newCTID
    $subfolder.Item.Update()
}

Documentation:

SPFolder members

SPListItem members