Preconditions:

1. Route all traffic through pgbouncer in transaction pooling mode.

2. Logically replicate from old to new.

For failover:

1. Ensure replication is not far behind.

2. Issue a PAUSE on pgbouncer.

3. Wait for replication to be fully caught up.

4. Update pgbouncer config to point to the new database.

5. Issue a RELOAD on pgbouncer.

6. Issue a RESUME on pgbouncer.

Zero downtime; < 2s additional latency for in-flight queries at time of op is possible (and I've done it at scale).