Sometimes when you have an existing MailChimp list, you need to add new group information to the list.

It is very hard to update many or all contacts in one go. 

ChimpySync lets you sync your contacts to Outlook - update them in Outlook, and then sync the updates back to your MailChimp list.

This video shows you how to do it.  If you want to download and install ChimpySync - you can do so from this page :