Converting 3 family to 2 family

[Laurence Fru]

Morning all, 

I'm in the process of looking into purchasing a 3 family with the intent of partial house-hacking. In truth, this will be more of a family home, so the plan would be to turn the 3 family into a 2 family - taking the top 3 floors and making them into a owners tri, and then having the basement unit as an Airbnb. 

Does anyone have experience with changing the designation from 3 to 2 family? As this would be one of my first major projects here, this is all pretty new to me. 

Are there any errors I can make with doing so? 

The current tenants in the property are on a month to month lease, due to the nature of the work I would be doing, I would likely be looking to purchase as vacant.