You can upgrade from the Free retail plan to the Plus plan by subscribing to it here. You'll be able to utilize all existing features and setup. The only new setup will be for the added functionality that's included with your upgrade. After your 30-day free trial, you'll be charged $60 per month per location. If you choose not to subscribe, you'll be downgraded to the Free retail plan.
To upgrade to the custom Premium plan, contact our sales team.