The email sent by Stripe says "Since you used Stripe Subscriptions in the past year, the Stripe Billing Starter plan is included in your current pricing."

So... based on that, it seems that "existing customers" means "customers that have actually used the product we're replacing".

I think we'd all appreciate a confirmation or clarification, though. :)

We'll make sure you're taken care of here. (If you're just running test subscription charges right now, you're still grandfathered. You can ping me any time if you have questions.)