I want to downgrade to a smaller Heroku Postgres plan.
The procedure for downgrading Heroku Postgres plans is much the same as the one for upgrading, as described here: https://devcenter.heroku.com/articles/updating-heroku-postgres-databases The only difference is that the new plan is of the smaller type.
Please note that downgrading a plan may result in fewer Postgres connections being available to the app. Also the memory limits are significant as they often have a direct impact on the performance of queries. You can find more about choosing the right plan for your application here: https://devcenter.heroku.com/articles/heroku-postgres-plans