In situations where the block time is subject to a hard cap, a Substrate chain needs to be able to execute a(n expensive) storage migration over the course of more than one block. This will likely happen in the context of running as a parachain on Polkadot, as validators will enforce a maximum block time on parachain blocks.

This can likely not be addressed by the existing task scheduling means as the blockchain is not operational while migrating. It should likely be "frozen" to avoid inconsistent states.