Gov Net GIG Know Project . The basic thesis of this book is that people want to change their buildings. So buildings do change.
Brand has lots of interesting pictures showing changes that have taken place in buildings.
In other words, it doesn't change very fast. They can be changed, but not easily. Changes in structure occur slowly enough to be in competition with the building itself being torn down. This gets changed on time scales of a decade. Technological changes lead to old buildings always being behind the times and getting service changes as money allows.