In parallel with the deployment of grid infrastructures in Europe, scientific user communities that use them to federate their computing and data resources have developed. These communities have been supported through a diverse collection of national and European projects, fostering technological change within their partner institutes. In many cases though, the patchwork nature of the funding and of the collaborations made broad technological change within those communities difficult. The transition from project-based grid infrastructures to a sustainable European Grid Infrastructure (EGI) presents challenges for the scientific user communities as well. User communities using EGI will be organized into Specialized Support Centres (SSCs) that will coordinate activities, safeguard technological expertise, and foster collaboration between people, institutes, and projects within the community. This presentation enumerates the challenges faced by the user communities during this transition and concludes with a discussion of how those challenges are currently being met.