Can You Migrate an Old Database Without Taking the Business Offline?
Usually, yes. If your team or vendor controls the application code, the expand-contract pattern changes a live database in additive steps: add the new structure, write to both, backfill history, move reads over, and drop the old structure last. Nothing goes offline and every step can be reversed. If you are replacing a system whose code you cannot change, a parallel run is safer. It costs extra weeks, but it removes the hard cutover.