When you create a server account, or a cloud account, the password you entered, including any private key information, is stored on the most secure app on your Mac: the Keychain Access. All the passwords used by the Migrate app are stored under the Keychain Access app, on your Mac.
If you want to delete all passwords, you can go to Migrate preferences and click on “Delete all passwords”. This action will remove all passwords from the Keychain Access app related to the Migrate app. If you try to migrate an account in a further stage that need that deleted password, the edit account view is presented in order for you to input the password again.