  With the rapid evolution of technological, economic, and regulatory
landscapes, contemporary blockchain platforms are all but certain to undergo
major changes. Therefore, the applications that rely on them will eventually
need to migrate from one blockchain instance to another to remain competitive
and secure, as well as to enhance the business process, performance, cost
efficiency, privacy, and regulatory compliance. However, the differences in
data and smart contract representations, modes of hosting, transaction fees, as
well as the need to preserve consistency, immutability, and data provenance
introduce unique challenges over database migration. We first present a set of
blockchain migration scenarios and data fidelity levels using an illustrative
example. We then present a set of migration patterns to address those scenarios
and the above data management challenges. Finally, we demonstrate how the
effort, cost, and risk of migration could be minimized by choosing a suitable
set of data migration patterns, data fidelity level, and proactive system
design. Practical considerations and research challenges are also highlighted.
