Coverting basement into cellar

I have a two family house with three floors. The CO lists the 1st and 2nd floor as “dwelling” use and the lower/”Cellar” floor as storage & Garage use. The house was built in 1940 and was built with a full bathroom in the “Cellar” as were the adjoining houses which were all built together. The ceiling height in the cellar is about 7’6″, the cellar has two means of egress to the outside, a front door and rear door, in addition to a stairwell to the middle floor, presently the middle floor and Cellar are utilized as one apartment. The ceiling height in the cellar is right around 50% above grade, I would have to get a architect/surveyor/ etc. To measure exact. If I wanted to change the CO to basement what I be facing? Any other input positive or negative would be appreciated. I am looking to utilize the lower level legally as livable space, and possibly make the lower level(basement if changed) a apartment, and utilize the top two floors as the other apartment.