Have current rails 5 heroku app - how do I start a new version on rails 7 without screwing up my current deployment?

I have a current Rails 5 app with my current database running on Heroku. What I would like to do is create a new version of my app using Rails 7, that uses a modified database (that adds some additional attributes). How do I set this up (maybe using a new branch of git), and get it all running locally before deploy to Heroku.

What is the best way to set this all up, without screwing up my current Heroku deployment?

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