In reengineering legacy code it is frequently useful to introduce a design pattern in order to add clarity to the system and thus facilitate further program evolution. We show that this type of transformation can be automated in a pragmatic manner and present a methodology for the development of design pattern transformations. We address the issues of the definition of a starting point for the transformation, the decomposition of a pattern into mini-patterns and the development of corresponding mini-transformations that can introduce these mini-patterns to a program. We argue that behaviour preservation is a key issue and develop a rigorous argument of this for each mini-transformation we discover. The architecture of an existing software prototype is also discussed and the results of applying this methodology to develop a transformation for the Factory Method pattern are presented.