Direct device-to-device (D2D) communication is considered as a key technology and enabler for a multitude of new services in future fifth generation (5G) networks. Especially safety and emergency applications will require fast and reliable handover, seamless D2D connectivity and proper radio access to the cellular network. This paper proposes a novel handover scheme, which takes advantage of the D2D sidelink in order to assist the handover procedure, provide connectivity and maintain synchronization among multiple radio links. The scheme is in particular beneficial for cases including users with partial cellular coverage, for which standard handover is not possible, provides connectivity to out-of-coverage users and further assists them in accessing again the cellular network. In a network-controlled implementation the proposed scheme can be used for flexible handover, allowing multi-link coexistence and avoiding large delays of inter-cellular D2D. Sequence diagrams and procedure flow charts are provided for selected scenarios.