A: This can only be done using the import-tool.
Set up the file and the import like this:
- Put the new email address in the column you connect to the contact's Email field.
- Add a column that identifies each contact, such as Contact: ID or the contact's name, and use it for matching. Do not use Email for matching: the import would look for a contact that already has the new address, and the row would not match.
- Add a company column, such as Company: ID or Company name, and use it for matching too. Contacts are only matched within the company the row matches, and the import does not continue with contact fields matched but no company field.
- Choose Update so that rows that do not match an existing contact are not created as new contacts.