For object oriented software development, evolution of-ten requires changes to the underlying class hierarchy structures of the program in terms of class, inheritance and association relationships, fields and operations. These revisions unnecessarily burden the implementation process and are due to the fact that some design information is only implicitly present in the class hierarchy. Whit this in mind, software engineers try to redesign and/or reprogram existing softwares with minimal modifications on the class hierarchy structures so that software can be refined and reused easily. This paper discusses the evolution of object-oriented softwares modeled by Object Implementation Graphs(OIG) which completely represents the class hierarchies from a programming language perspective using nodes, arcs and labels. Based on the OIG, we present a set of lower and higher level primitive transformations that provide a means for management and useful guidelines of object-oriented software evolution. The result of this research leads to a better understanding of evolution and reuse of object-oriented software and class hierarchies. It also provides a mechanism for making the changes of class hierarchy easily and safely.