Heroku postgres create follower db and promote follower to master from slave

I needed to migrate heroku postgres between 2 different heroku applications. One of them has heroku postgres add-on, but the other was connecting to first application's postgres.
However, I wanted to delete first application, so here is how I migrated heroku postgress add-on to another application.

NOTE: you cannot use follower db with hobby plan

1. Create new postgres on second app which follows original db

   heroku addons:create heroku-postgresql:standard-0 --follow postgresql-test-xxxx -a second_app

   you can check add-on name with

   heroku pg:info -a first_app

2. Check how much new db catch up original

   heroku pg:info -a second_app

   This returns

   Following:   HEROKU_POSTGRESQL_LAVENDER (DATABASE_URL)
   Behind By:   125 commits

   Once behind by is 0 commits, you are ready to promote!!

3. Turn on maintenance mode to avoid database update(this cause downtime, but should be less than 1 min...)

   heroku maintenance:on -a first_app
   heroku maintenance:on -a second_app

4. Unfollow the original postgres

   heroku pg:unfollow HEROKU_POSTGRESQL_COLOR_URL -a second_app

   (this url is new postgres url. you can get with heroku config -a second_app)

5. Promote new postgres

   heroku pg:promote HEROKU_POSTGRESQL_COLOR -a second_app

   This update config DATABESE_URL with new postgres URL

6. Turn off maintenance mode

   heroku maintenance:off -a first_app
   heroku maintenance:off -a second_app