What stays the same
Your code. ShipSanely reads the same repository; you do not add an SDK or rewrite configuration. If your app ran on a PaaS it almost certainly runs here.
What you can delete
Platform-specific config files, buildpack hints and most of the deploy script. Keep any migration tooling — the agent uses it.
Step 1 — connect and plan
Connect the repository and review the plan. Confirm the services and datastores match what you run today.
Step 2 — move the data
Provision the new private PostgreSQL, then use shipsanely db tunnel with pg_dump/pg_restore for the bulk copy. For near-zero downtime, run a logical-replication catch-up and cut over at a quiet minute.
Step 3 — deploy and verify
Deploy to the hosted endpoint while your old platform still serves traffic. Test against the real domain using a hosts-file override.
Step 4 — switch DNS
Point your domain's CNAME at the hosted endpoint. TLS is issued automatically. Keep the old platform alive for a day, then decommission.
What you gain
Verification before every promotion, private datastores, an agent on call, and a bill split into two meters you can explain.