What happens to active subscribers when you run a membership giveaway?

We’re looking to run a promo campaign to give out free membership months but I'm stuck on how to handle people who are already paying us every month. Manually pushing back their renewal dates works for now but it gets messy fast. Do you just stack the free time on top of their current plan convert it into in app credit or hold off until their subscription ends?

How are you all handling this without creating a nightmare for your payment setup?