Multi family zoning

Hello, I'm in the process of possibly purchasing my first investment property and I'm extremely nervous. They property is currently listed and used as a duplex but was zoned as a quad. Two of the units are currently rented and the plan is for me to purchase the property and re-zone it as a quad and occupy one of the remaining units. My question is has anyone ever dealt with anything like this before and if so, what should I look out for? Thanks for any help/input in advance.