PowerShell: Modify Content Type in specific sub folder

I am using the following power shell script:

http://get-spscripts.com/2010/10/change-content-type-set-on-files-in.html

To change the content type from the standard “document” content type to a new one in a document library, This works fine but i have a requirement that i only need to apply a certain document type to documents and documents in sub folders which are only in a certain root folder in the document library.

This has stump me on how you tell in to only look in a certain folder, I belive is something along this line: I need to tell the powershell script to select the folder like this, but what ever i try does not seem to work.

list[“Documentlibaryname”].RootFolder.SubFolders[“Folderwhicholdtheitemiwanttochange”]

but then i thought it might not work like that i might have to do some looping of folder names like in the follow link to select the correct folder:

Get all document from all folder and subfolder from document library sharepoint powershell