This article discusses the fluidity of ethnic categorization in the context of changing ethnic relations, based on a diachronic and regional approach. It is argued that such an approach is needed to understand identity trajectories of peoples of the Sino-Tibetan borderlands. While studies on minorities in China are often limited to one ethnic group, the perspective defended here considers the interactions between several groups in specific contexts, and at different scales, which contribute to determine shifts in identity. The analysis focuses on the Sino-Tibetan borderlands of today’s Sichuan and Yunnan provinces, and the diverse and often Tibetanized groups located in these mountains. Taking into account the changing socio-political environment, mainly from the late Qing to the People’s Republic of China, helps us to uncover a number of factors determining the identity formation of the groups under consideration, and the persistence of a form of genealogical thinking. A range of cases is considered to provide comparative data at the regional level, supporting the degree of generality of the lability of ethnic categorization as well as identity changes, and the disjunctions between assigned categories, culture, language, and local identity.