As systems evolve, they become harder to understandbecause the implementation of concepts (e.g. businessrules) becomes less coherent. To preserve source codecomprehensibility, we need to be able to predict how thisproperty will change. This would allow the constructionof a tool to suggest what information should be added orclarified (e.g. in comments) to maintain the code’scomprehensibility.We propose a framework to characterise types ofconcept change during evolution. It is derived from anempirical investigation of concept changes in evolvingcommercial COBOL II files. The framework describestransformations in the geometry and interpretation ofregions of source code. We conclude by relating ourobservations to the types of maintenance performed andsuggest how this work could be developed to providemethods for preserving code quality based oncomprehensibility.