When an organization faces new types of collaboration, for example after a company merger, there is a need to integrate the existing software. Two main process challenges are how to arrive at a realistic vision of a future integrated system, and how to actually carry out the integration process. We have performed a multiple case study, consisting of 9 cases. This paper presents the observations made in the form of recurring patterns that can be used as recommendations for other organizations facing the same challenge. Also discussed are the similarities and differences between already known software process best practices and the integration patterns found.