expand-and-contract

Replace a system with Parallel Change, also called Expand and Contract. Build the new structure beside the old one, migrate existing usages, then delete the old structure. A later slice waits until production has reached its checkpoint. An additive migration must be in production before code that uses the new schema deploys. A version that has stopped using the old schema must be the running version before a destructive migration removes it. Use when yak-shave has confirmed the capability set and the interface, or when the user asks for parallel change or expand-and-contract and that set is already decided. Do not use to choose which capabilities to retain, retire, or introduce (yak-shave), to harden a sound architecture (fix-loop), to choose among greenfield designs (design-space-exploration), or to release or deploy.