In this case study we examine sound change of Altoberdeutsch <iu> in Swiss German dialects. We used contemporary dialect data from nearly 60,000 speakers – collected with the smartphone app Dialäkt Äpp – and compared it to historical Atlas data from the 1950s. Results revealed hierarchical and contra-hierarchical diffusion patterns for some dialectal variants, while other variants remained virtually unchanged over the course of seven decades. We further report change in apparent time, with older speakers using traditional variants more frequently than younger speakers. Using this case study as a model, future work using the Dialäkt Äpp corpus will reveal patterns of feature diffusion and dialect leveling on a larger scale.