I called FinCEN with this question a few months ago and was told that the business could stop being an MSB by adopting a policy that it would not cash checks in excess of $1,000 in the future and adhering to that policy.
A few weeks later, I had lunch with an IRS MSB expert (the IRS is the primary regulatory agency for MSBs) in Miami. She said that was not true. She said, "Once they become an MSB, they are an MSB forever."
I'm doubtful about the "forever" part, but do believe it makes more sense to say that they cannot change from an MSB to an "un-MSB" just by performing an act of contrition. The FinCEN representative indicated they were going to issue guidance on this point, but that has not happened in the interim.
I suggest you call FinCEN, get your own answer and document the converstation if you do not intend to require them to register. Please note, your monitoring responsibilities do not change appreciably either way.
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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.