Issue
I want to downgrade to a smaller Heroku Postgres plan.
Resolution
Heroku supports three methods for changing (including downgrading) the plan of your Heroku Postgres database.
See Changing the Plan or Infrastructure of a Heroku Postgres Database for specific steps depending on your current database plan.
Note that downgrading your database plan may result in fewer Postgres connections being available to the app. Downgrading your plan to a smaller plan that offers fewer database resources such as memory or vCPU can have a direct impact on the performance of your database queries.
See Choosing the Right Heroku Postgres Plan for details on the plans available and their different tiers and features.
You can find more about choosing the right plan for your application here: https://devcenter.heroku.com/articles/heroku-postgres-plans