You need to explain this situation to the support offered from McAfee. They have the ability to deactivate a serial number and then generate another one that can be used with your version of the software. Of course, they need to verify that you are indeed a customer of a McAfee product, then everything should be easier.
You can contact the developers using the following link: http://www.mcafee.com/us/about/contact-us.aspx
Note: Make sure to explain exactly what happened and provide the details you've used when you purchased it.