One of the most common questions with existing buildings is: “What do I have to do if I change the occupancy of the space?”
The International Building Code specifically addresses this circumstance. It states that changes of occupancy and use require you to comply with the requirement of the new occupancy. This is true even if you are moving to a different division of the same occupancy. If you are moving to a less hazardous occupancy and you do not meet the requirements of that occupancy, you may be able to get approval from the building official.
Stairways in an existing structure do not have to comply with the requirements for pitch or slope. If you need additional stairways, they are still required.
In the 2009 IBC changes of occupancy must always meet seismic requirements. This is not required in 2006 and prior codes.