Some systems support Document Categories and others support Folders.

--When a document has a category, prepend the folder with the document category.
UPDATE 
    __M_Documents_Digital_Files
SET
    --Prepend the Category
    Final_Folder_RelativePath = CONCAT('\', V2.Final_Subject, Final_Folder_RelativePath)
FROM
    __M_Documents_Digital_Files V1,
    __M_Documents_Digital_Categories V2
WHERE 1=1
    AND V1.Final_DocumentCategory_Id = V2.Id

    --Only Prepend it where there is no folder
    AND Final_Folder_RelativePath = '\'

    --Don't prepend the category if it has already been prepended.
    AND CHARINDEX(
      CONCAT('\', V2.Final_Subject, '\'),
      V1.Final_Folder_RelativePath
    ) = 0


--Replace \\ with \
UPDATE
    __M_Documents_Digital_Files
SET
    Final_Folder_RelativePath = REPLACE(Final_Folder_RelativePath, '\\', '\')