Open Adaptive Complex Systems – such as road vehicle platoons or fleets of cooperative robots – may use dynamic reconfiguration to adapt to system or environment changes. One approach enabling this feature is Service-oriented Reconfiguration, where new configurations are created by composing the available services in an unconstrained manner. Due to the high number of possible service compositions, not all configurations can be pre-assured at design-time. Despite recent progress, there is no satisfactory approach for specifying safety cases in support of their re-evaluation at run-time, after system reconfiguration. To this end, in previous work, we introduced Dynamic Modular Safety Cases (DMSC). A DMSC is a modular safety case, which can be dynamically re-constructed and re-assessed given service reconfiguration. In continuation of the previous work, in this paper we provide guidelines for specifying safety cases at design-time, whose modular structure mirrors the system service decomposition, to enable their re-construction and re-evaluation at run-time in the event of a system reconfiguration. Aiming to support the specification of DMSC, we extend FASTEN, an engineering tool for the design and verification of safety-critical systems. We exemplify the specification of DMSCs in FASTEN for an illustrative example from the smart factory domain.