Change github repo for deployed heroku project

Question

I am looking to create a new portfolio. The current one is just using nodejs / express and the basics. It is already deployed on heroku. I would like to maintain / keep the same portfolio project link.

Where I have a question:... I either want to add react to existing portfolio and refactor/enhance or create a new react app and deploy to heroku with the same project name / deployment. My research says I can't fully add react to existing project. (Please correct me if i am wrong). Am I wrong? If i am right.. I would create a new react app / portfolio and then want to change the github repo associated with the heroku project? Can I do this? I could not find the info in their docs. THANK YOU!!