Most legacy-warehouse migrations do not fail because the target platform is wrong. They fail on the unglamorous parts: nobody trusts the new numbers, the cutover drags on for a year, and the business loses patience. After enough of these, you learn that migration is a trust problem wearing a technology costume.
1. Assess before you move anything
Inventory what you actually have: tables, pipelines, reports, and — critically — what is genuinely used. A huge fraction of legacy objects are dead. Migrating them is pure waste. Rank what is left by business value and by conversion difficulty, and sequence the work so early wins build confidence.
2. Convert in waves, not in one big bang
Pick a thin, self-contained slice — one data domain end to end — and move it fully: ingestion, transformation, and the reports on top. A big-bang cutover concentrates all the risk into one terrifying weekend. Waves let you learn cheaply and keep the lights on.
3. Reconcile to the cent
This is where trust is won or lost. Run the old and new systems in parallel and reconcile outputs automatically — row counts, sums, and the handful of executive numbers everyone actually watches. When a VP sees the new platform match the old one to the penny for a month, the political battle is over.
4. Govern from day one
A migration is the rare chance to fix what the legacy system never had: clear ownership, access control, lineage, and cost visibility baked in from the first table — not bolted on after. Do not rebuild the old mess on better infrastructure.
5. Cut over, then decommission deliberately
Switch consumers to the new platform once reconciliation holds, keep the old system read-only as a safety net for a defined window, then decommission it on purpose. An un-decommissioned legacy system is just double the cost.
The technology is the easy 20%. The reconciliation, sequencing, and trust-building are the 80% that decide whether the migration ships.
Our flagship course The Databricks Migration Architect walks a complete legacy-EDW-to-lakehouse migration in this exact sequence. Explore it here, or book a migration review to pressure-test your plan.