If you want to change the price of your paid newsletter, you can do so by adding a new price to the existing product in Stripe and archiving the old price. Buttondown will automatically use the new price for new subscribers; existing subscribers will retain their current price.
Migrating existing subscribers to the new price is more complex: neither Buttondown nor Stripe currently supports this within the interface. We are happy to run this manually for you; please contact support if you'd like to do this.