Select the old account, right-click it, select Settings. The Account Settings panel will appear. At the bottom, the Account Actions button will offer options to remove and add accounts. You say you have forwarded messages from the old account to the new. That word forward is ambiguous. If you mean you have re-emailed the messages to yourself, you didn't need to do that. You could move them within Thunderbird - the Local Folders account is provided for this. Re-emailed messages will typically appear to be from you and to you, so you lose their context and provenance. If you have set up a server to forward messages, how confident are you that this will continue after the demise of the old account?