The article I have uses DFDM to edit specific records and seems to match closely the 20911 version you have. Because key changes need to be made to two of the files, my thought was to export all 4 files listed in KB article 20911 to CSV, make the changes to each using Excel or an editor, import them all back in and rebuild the sort files. In our case, the whole customer base is set to BF.
It sounds like even the manual methods haven't worked for you. I would expect the aging to be a bit weird until the converted BF Invoices have all been paid.