Nested state diagrams are a commonly accepted design notation for modelling complex software systems at different levels of detail. Besides nesting, other layering mechanisms are needed. However, incremental development of layered state diagrams raises the problem of change management. Changes made at high abstraction layers can have a serious impact on more concrete layers, as conflicts can be introduced that are propagated through all layers. To resolve these conflicts, the methodology of reuse contracts is applied. Carefully choosing a set of incremental modification operators and investigating their interactions allows us to categorise the conflicts. These conflicts can be detected semi-automatically, facilitating incremental design.