Previous studies investigating how the school choice paradigm shapes school segregation have found that students' ethnic school preferences drive school segregation by leading students to rank and change current schools following ethnic homophily orientations. This study investigates an intermediate moment in which these preferences contribute to the exacerbation of school segregation: students changing schools after being allocated to following admission rules but before the start of the academic year. We refer to these changes as micro-changes. Using Swedish register data on 9th-grade students applying to upper secondary education in Stockholm schools, this study evaluates how micro-changes affect school segregation. Our findings reveal that micro-changes are not neutral and increase school segregation levels because (1) students tend to reject of schools with a low share of in-group members and low representation of 9th-grade classmates, and (2) micro-changers move into schools with a high share of in-group members and 9th-grade classmates. Furthermore, our simulation model shows that micro-changes impact on school segregation have a cumulative effect.