Trendyol's product content team details a phased, zero-downtime migration of five 'helper bucket' datasets (2.24 billion documents across two clusters) from Couchbase to PostgreSQL to cut infrastructure costs while preserving read performance. The strategy combined dual-writes via Kafka consumers, a custom CDC-based migrator tool for historical backfill, and feature-flagged cutover of read traffic. Key production challenges included a mysterious TPS spike traced to PgBouncer's server_reset_query_always setting, a connection-limit incident causing a primary node failover, and a hidden scaling issue where PgBouncer's own horizontal autoscaling multiplied backend connections beyond expectations (20 pods × 150 default_pool_size = 3000 potential connections). Final production benchmarks showed stable P50 latency and only a brief P99 spike (200ms to 400ms) under peak load, validating the migration.