Connection slots reserved error

The API runs in us-east-1, us-west-2, and eu-west-1 behind latency-based DNS. A config change last Tuesday raised the DB connection-pool max from 50 to 200 per pod to handle a planned campaign. The campaign launches today. eu-west-1 is fine. us-east-1 and us-west-2 both start throwing FATAL: remaining connection slots are reserved for non-replication superuser connections under load; p99 spikes, then partial outage. Dashboards: each region's pod count and per-pod pool size look identical in the deploy manifest, but the Postgres max_connections differs, eu-west-1's primary was resized to a larger instance class last quarter (max_connections 800), the other two are still on the old class (max_connections 200). How do you triage and mitigate?