This paper proposes to integrate the diachronic dimension to the typological study of gender complexity, and focuses on the morphosyntactic encoding of gender distinctions via agreement patterns. After investigating the processes of language change that foster the reduction, loss, expansion and emergence of gender agreement in a sample of fifteen sets of closely related languages (N= 36 languages), we discuss how gender agreement systems in decline and on the rise pattern in terms of complexity. We show that declining and emerging gender agreement systems may exhibit increase or decrease in complexity and discuss how this relates to the fact that they represent transitional stages between absence of gender and full-fledged gender systems. In our analysis, we make use of typological implicational hierarchies in the domain of agreement as a tool to account for diachronic variation and for the patterns of simplification/complexification in the agreement systems of the sampled languages.