SharePoint 2013 conversion from MSDN license to fully purchased license

I am seeking to apply a fully licensed product code on a SharePoint 2013 MSDN in the 'change farm license type' in central admin, but the area to add the product code is inaccessible.  

Is it possible to upgrade a MSDN install without full re-install of SharePoint 2013?

please advise me on this.

thanks

kwade