Thank you for reaching out to Constant Contact API Developer Support.
A 409 conflict usually comes up when you are trying to use something that is unique and already in use, such as an email address. Though I can't say for certain why you are seeing this on an individual update when making no changes at all.
Since it seems to only be this one contact, I'd like to get more detail about them. Please email us at webservices(at)constantcontact.com with your Constant Contact username, and the email address for this contact and we'll be happy to take a closer look.