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Tools available for measuring the modifiability or impact of change of a system through its architecture typically use structural metrics. These metrics take into account dependencies among the different elements of a system. However, they fail to capture the semantics of an architectural transformation necessary to control the complexity and cost of making changes. To highlight such limitations, this paper presents a study where we applied a representative structural metric, called 'propagation cost', to archetypical architectural transformations known to affect system modifiability such as rearchitecting a tightly coupled system to a layered pattern. We observe that in its original form the propagation cost metric does not provide consistent indications of architecture health. Enhancing this metric based on the semantics of the architectural pattern and tactics used in the transformation show improvements. Our results demonstrate that these enhancements detect modifiability properties that are not detectable by the propagation cost metric.