the Rotary Speaker already does this
That's a very nice demo of the transition effect, Don. I'm not sure how to set that up with my midi foot controller (Ground Control Pro - any help here would be great).
But my real point in suggesting variable fast and slow speed controls is that we are playing guitars here, right? We don't have to emulate an organ's transitions to be creative with this nice effect. Having total control over the speed parameter, as in so many other effects on the KPA, would only enhance it's value imho.