FB08 is "Reverse Document" so changed behavior "some" years ago, AFAIK it does no longer build a new document with reverse amounts, but use the current document to be reversed, so you can no longer change most of the fields.
Hints: If I remember (46A?) look for OSS notes with new cancellation procedure keywords.
Also those documents are immediately cleared, so why do you want to change those payment block fields, are you able to change those with FB02 or FB09 ?
Regards,
Raymond