The building code sorts the ways that buildings are used into separate "occupancy classifications". Every building is given an occupancy classification when it is built, and each occupancy classification has different building code requirements that go along with it. A "change of occupancy" is when there is a change in the building's use that would place it in a different occupancy classification.
A change of occupancy for an existing warehouse will require that the building meet most of the current building code requirements for a new building. Whether the proposed use of the warehouse is manufacturing, office, retail, etc, this will likely mean adding more exits, emergency lighting, or other safety improvements. You may also be required to add more restrooms to serve the increased number of occupants.