RDS production Instance downsize

Hello.

If you do not expect high loads to occur on RDS in the future, you may want to change to the next lower instance type.
However, if you cannot predict the future load situation, I think it would be safer to leave some leeway in the specs, so I think you can leave the instance type as is.
If you enable Multi-AZ configuration, you can reduce downtime when changing instance types, so even if you want to return to the original instance type after downsizing, you can quickly change it.
https://repost.aws/knowledge-center/rds-mysql-downtime-impact

If you can afford to migrate RDS PostgreSQL to Aurora PostgreSQL, you may want to consider using Aurora Serverless.
Aurora Serverless automatically changes specifications within the specified ACU (spec) depending on the load situation, which may lead to cost reductions.
https://docs.aws.amazon.com/AmazonRDS/latest/AuroraUserGuide/aurora-serverless-v2.html