This article explains how to change or reset a FortiCloud account id.Solution
Run the following commands on the FortiGate CLI:
config system fortiguardLogout from the ForitiGate and log back in after few seconds.
After the new login to the FortiGate, proceed to re-register FortiCloud with the new email ID.
Go to FortiGate GUI > License widget > Activate FortiCloud > create a new account, enter email ID (the new value that is to be used) and password.
Check email inbox, when email is received click on the embedded link to confirm.