Subdividing in Boston

[Stephen Daddario]

Hi- I have a 2 family on a large lot that I would like to subdivide. Does anyone know what happens to the existing 2 family if after I subdivide it? It  now becomes non conforming due to the side set back. Would that mean I have to take the existing structure through zoning? Any ways around this... not sure if I could get a permit to rehab/ sell it since it is now non conforming structure. 

Any insight is much appreciated.