Formal methods have had little impact on software engineering practice, despite the fact that most software engineering practitioners readily acknowledge the potential benefits to be gained from the mathematical modeling involved. One reason is that existing modeling techniques tend not to address basic software engineering concerns. In particular, while considerable attention has been paid to the construction of formal models, less attractive maintenance issues have largely been ignored. The purpose of this paper is to clarify those issues and examine the underlying requirements for change support. The discussion is illustrated with a description of a change technique and tool developed for the formal notation LOTOS. This work was undertaken as part of the SCAFFOLD project, which was concerned with providing broad support for the construction and analysis of formal specifications of concurrent systems. Most of the discussion is applicable to other process-oriented notations such as CCS and CSP.