How would you go about changing a 2 family house into a 1 family for mortgage purposes?

A brownstone 3 story family in brooklyn was bought as a 2 family home. Now one family lives there and there is nothing visible that distinguishes it as a 2 family home. If they needed to refinance how do they go about changing it from 2 family to one family?How would you go about changing a 2 family house into a 1 family for mortgage purposes?You'd contact your city/county zoning authority to try to get the zoning changed. This could take a while, if you're even successful. Most properties are worth more zoned for multi-family, so I'm not sure it's such a good idea anyway.
How would you go about changing a 2 family house into a 1 family for mortgage purposes?
What distinquishes it is all the property records and a title search. Do you think the finance companies aren't going to check that basic fact?