This study is the first to explore microdevelopment in sociolinguistic evaluative judgments of standard German and Austro-Bavarian dialect by adult second language learners of German by using dense time serial measurements. Intensive longitudinal data (10 observations per participant) were collected from four learners at approximately weekly intervals over 3 months. We employed generalized additive models with superimposed periods of significant change to identify rapid developmental phases in individual developmental trajectories. By triangulating these models with qualitative introspective and retrodictive interview data, we identified environmental and psychological stimuli for change. Learners evinced increasing and decreasing periods of significant change, independent of length of residence. Dynamic constellations of identity- and agency-related variables alongside more intensive social interaction with target-variety speakers contributed to significant changes. We discuss findings from a complexity perspective and advocate for microlongitudinal studies in variationist second language acquisition to better capture stimuli for change in learners' emerging multivarietal repertoires.