How to remove a subscriber from list #1 and add to list #2

Hi,

I am trying to find the best way to integrate a WooCommerce + Sendy however I am missing a few features like:

1. Visitor subscribes via gravity forms + gravity forms addon (becomes a subscriber)
2. Subscriber buys something after a few days and becomes a buyer (remove the subscriber from list #1 and add to list #2)

How do I do this?

Thank you