While case grammars have frequently been used to implement natural language interfaces to various sorts of systems, there has been little agreement over what precisely the different cases ought to be, or exactly where the semantic boundaries between different cases ought to be drawn. As a result, the domain specific decisions taken in the implementation of a case based natural language interface can result in a system which is very difficult to modify to meet the demands of a different application domain. This paper describes an approach to analysing the case grammar structures used for particular domains and utilizing these analyses to facilitate the porting of case based natural language interfaces from existing domains to new ones. The analysis is based on the assumption that there is not a single correct set of cases that is applicable across all of language and world knowledge, but rather that a set of basic cases is transformed somewhat by the semantic requirements encountered in any particular domain. A type hierarchy of cases is developed consisting of three levels of generality. The lowest, most specific, level of the hierarchy consists of a domain specific mapping of the general cases dictated by the semantic requirements of the domain. The analysis procedure consists of explicating the mappings for the two domains and comparing them in order to discover the necessary shifts in representation required to capture the shifts in domain semantics.