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.
Even today, office (B) and retail (M) occupancies are classified at the same "hazard level", and the building code requirements are very similar. The Bureau of Development Services has a Fire and Life Safety Guide for Existing Buildings that allows a change from retail to office, or office to commercial, to be reviewed more like a remodel than most changes of occupancy.
Call a Life Safety Reviewer at (503) 823-1456 with questions.