I too converted to a paid database. As part of this process Heroku gives you a config variable for the new paid database.
Free vs Paid Config variable
In my app I had referenced “DATABASE_URL”, so I had to go through my app and change all references from “DATABASE_URL” to “HEROKU_POSTGRESQL_GREEN_URL” which is the new config variable for the paid database.
Locating config var names
Select your app then select “settings” and it’s in a section called “config vars”
The files I needed to modify in my phoenix application to were config/prod.exs and lib/yourappname/repo.ex.
The feedback in this forum is correct just leaving more details for any newbies to Heroku deployment.
