If I purchase a web site license can I later upgrade to the web server license?  Yes, you can upgrade at any time from a web site license to a web server license for the difference in price between what you paid for the web site license and the current price of the web server license.

You will need your original orderid (sent with your web site license purchase) in order to place an upgrade order.  Upgrades can be made at https://www.dotnetcharting.com/store/upgrade.aspx