I had no idea what the answer to this question was, but it got me curious too so I started messing around with my site. Here's what I figured out:
I changed my HURLs from [[category]]/[[title]] to just [[title]] Then I tried loading my original HURL: http://skunk.guru/main/skunk-colours-and-patterns/ It still loaded.
I tried the new HURL: http://skunk.guru/skunk-colours-and-patterns/ and it loaded as well.
In the Green domains references I now have two references set to dynamic.
This leads me to believe Google will see these HURLs as duplicate content (which is something I would not want). So I tried testing the button in Green Tools that says "clear HURL cache" and I still see both of them. Toggled HURLS to No and back to Yes. Still see both of them.
My conclusion: Unless you want to clear the old HURLs out manually, or someone knows a way to set those to stale or unused, I would avoid changing this after HURLs are created just to avoid the duplicate content URLs for your old items. If you don't have a lot of old items, and don't care, I made a new webpage and it only created a HURL using the new setup ( [[title]] ).
Hope my messing around helped answer your question, and if anyone knows how to clear out old unwanted HURLs please feel free to chime in.