This has come in handy more times than it should have.
To redirect all requests to example.com:
heroku config:add NEW_BASE_URL=http://example.com
To use an alternative HTTP status code set
REDIRECT_STATUS. Defaults to 302.
heroku config:add REDIRECT_STATUS=301
If the old app that you're redirecting from isn't a Node app you'll need to change the buildpack:
heroku buildpacks:set heroku/nodejs
You can then replace the app by pushing with
git push heroku master -f
NOTE: If the above doesn't work you may need to delete and re-create your Heroku app before pushing an entirely new Git repository into it.