I would recommend documenting that conversation VERY thoroughly in your exemption account file. You will likely have to explain it to a number of auditors and regulators in the future. But that's just me. I generally follow the path I think will ultimately be the easiest.
I think that I understand the reason for the response. If the nature of business has not changed, then why would you need the 12 month history since your already have a transaction pattern established. That being said, I still would revoke the exemption and wait the 12 months. A DBA is a natural person. When a business entity is created, a new "person" is created. I wouldn't want to have to justify allowing the "person" to change and the exemption to continue.
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Life without Jesus is like an unsharpened pencil - it has no point.