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. Until the early/mid 90's, the building code didn't really distinguish between the retail and office uses and they were classified as the same occupancy.
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 typically requires that the building meet most of the current building code requirements for a new building.
Call a Life Safety Reviewer at (503) 823-1456 with questions.