You have changed your email, have a new FirstRain account created and want to merge it with the old account.
Unfortunately, there is no functionality to merge accounts in FirstRain.
As a workaround, submit a request to FirstRain Support to change the username and email for your old user account to the new email address; in this way, you will have access to all your configured channels with the new email address.
If a user requests to merge accounts, propose to change their email and username as described above.
If the user agrees, you will need to change its email and username by adding the prefix: deleted_ (or similar), then update the email and username for the old account.