contacts migration from windows live mail has put email addresses in the nickname field, how can I migrate all nickname fields to email fields?

Windows 8. moved from windows live mail email client as no longer supported. Exported contacts to folder. installed Thunderbird and imported the contacts from that folder. All looked OK except that I couldn't make it autofill the 'to' field so had to write out email address in full, if I could remember them. Eventually discovered that during the migration process, the email addresses had all been put into the 'nickname' field, and the 'email address' fields were all empty. Is it possible to move all the info from nickname fields to the email fields in one go, or do I have to go throught the whole contacts folder and do it all by hand individually?