In IOs where formal reform is difficult to achieve, informal changes may be much more pervasive. Even when formal rules remain the same, IOs need to accommodate various demands for change, especially when IO composition and IO relations with non-members are deeply contested. By combining historical institutionalism and practice theory, I look at the UNSC and zoom into the content, actors, and timing of institutional change. I uncover an empirical reality of astounding informal changes affecting not only UNSC procedure, but unexpectedly its composition. By practice tracing the codification of the UNSC working methods I show critical shifts in the UNSC composition, such as informal extension of the mandate of its elected members and permanent issue-specific access for troop-contributors to UNSC decision-making on peacekeeping. This type of institutional change- institutional stretch(ing) is both an outcome and a process. As an outcome, it encompasses not only informally changing IO procedure but also redesigning IO de facto composition by layering new membership configurations which can be more or less formalized, ad hoc or permanent, generalized or issue specific. By stretching beyond what formal rules allow, IOs respond to evolving realities and include relevant actors. As a process, change-making routinely happens beyond formal IO borders within a broader ecosystem of relevant actors created by the IO community of practitioners and their practices. While renegotiating contested IO properties on an ongoing basis, actors within the IO ecosystem use resources such as legitimacy, troops, money, and knowledge to become important players within the battles for bigger access.