Hello Karl!
I'm sorry but it is not possible to purchase the upgrade from one version to the other, the are part of two different editions of the software and it is not possible to upgrade to WebSite X5 Professional from any other version of the software.
Please feel free to ask me anything about the two versions and I will be happy to help you. Below you can find all the new Professional 10 functions

http://www.websitex5.com/en/highlights-professional-10.html

http://www.websitex5.com/en/functions-professional-10.html

http://www.websitex5.com/en/webhosting-professional-10.html

You can download the demo version as well from the following page if you wish to test it http://www.websitex5.com/en/demo-professional-10.html

Thanks,

Samantha