It's quick and easy to add a discount code to your existing plan. To do this, first you'll need to hit cancel on your plan:
Don’t worry about the cart deletion, this only happens if the plan remains cancelled for longer than 3 days.
Then go to activate plan again and in the top right corner of each Plan box, you’ll see the plus sign:
When you click on this, you'll see an option to add your discount code.
You'll then see your new monthly pricing, which you can accept.
During this process, your carts reverted to draft, so make sure to make your cart Live again after the plan with discount code is activated.
Please let me know if you need any further help!