Pro Plugin Distribution

I’m hoping someone can point me in the right direction on this one…

Question: How does updating work for Pro Plugins?

In more detail… I have a free version of my plugin hosted in the repository, so users are notified and prompted to update whenever I make updates to the trunk in the repository. Now I want to distribute a Pro Version of the plugin with advanced features, which I’ll host on my own site. But how will users be notified and prompted to update when I release updates? I can’t find any information on how to do it? What am I missing?

Thanks in advance for any help.