You are figuring it out. An individual who incorporates his business is a new customer, the corporation. To acknowledge that, my suggestion would always be that your new customer should observe the formalities of the corporation he as paid to establish and open a new account, not file maintenance the old one. As you note, you can wait two months or you can do a risk assessment in order to qualify your customer as an exempt person.
However, what you cannot do is bypass the requirement for the 5 large currency transactions. Apparently, there was some debate within FinCEN on why that would make any sense, but the way the regulations are written and the examples used in the supplementary information confirm that the "frequent transactions" are a condition precedent to gaining exempt person status.
As an aside, I think you are describing the best example for using the "risk assessment" option. You know this business and its owner... Write down what you know. Nevertheless, if it takes him a while to generate those five transactions you might just want to wait two months and spare yourself the increment in compliance risk. No one knows what imaginations field examiners will exercise when they evaluate these risk assessments...
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In this world you must be oh so smart or oh so pleasant. Well, for years I was smart. I recommend pleasant.