I have a Django app on Heroku that uses 2 dynos, 1 web and 1 worker(Celery background tasks).
I want to upgrade from free to hobby so the server doesn't sleep and the background tasks don't stop.
Should I upgrade the web dyno too? Or upgrading the worker dyno is enough?
You can't just upgrade the worker dyno. They're either all free dynos or all hobby dynos. From the Heroku docs:
If an app uses free dynos, it can use only free dynos for all of its process types.
If an app uses hobby dynos, it can use only hobby dynos for all of its process types.
Another option is to use an uptime service like Pingdom or FreshPing to keep your free web dyno alive, but you likely don't have enough free dyno hours to run both dynos 24/7.
So you probably need to upgrade both to hobby.
