How to change default sender address in address book

I'm in the situation where my pop3 email provider (Vodafone) is withdrawing their service, so have had to change to a new provider (in addition to using gmail and hotmail addresses). I have set up the new account (provided by Octopoly) on Thunderbird in the devices required. It works. There are two users in the household. I can not find the setting or option that allows our sender email address to automatically be changed when creating mail from the address book. I've tried exporting the address book from the original account, then importing it to the new account. It retains the original sender address, however. How do I do this? Do I have to edit the entire address book?