Spillway Pro is sold through the Mac App Store, so your subscription lives in your Apple Account settings — that is where you change plans, cancel, or check your renewal date. You can open it from Spillway: Settings ▸ Account ▸ Manage Subscription.
Your subscription
Spillway has no account to create and no password to remember. Pro is tied to the Apple Account you bought it with.
Sign in to the same Apple Account, install Spillway, then use Restore Purchases in Settings ▸ Account. There is nothing else to transfer.
Because there is no Spillway account, there is no account record to delete. Your notes and captures are stored on your Mac and in the vault or providers you configure, so removing them is entirely in your hands. For questions about data Spillway holds, email [email protected].