You cannot select Element Switches in a CONTROL SET when the function is a "COMMON" function. COMMON here literally means "all Elements in common/ as in together".

Often, I would like to use a Preset voice as a template for building a new voice, but if the Common Destinations are disabled for certain Elements in the Control Set, I may to re-enable them. So far, other than Initializing the voice and starting from scratch, I haven't found a way to enable Element Switches that were not enabled in the original Preset voice for the Common Destinations.

Common Destinations are always disabled for the individual Elements. The parameter if selected as a Destination will be applied to ALL ELEMENTS whether the grey-out check mark appears or not. That is a fact so nothing unnatural occurs.

You can enable the Element Switches by selecting a parameter that can be set individually by Element.

Volume, Reverb Send, Chorus Send, and the like is all Elements together.
C-LFO is the COMMON LFO all Elements arrive there together, you can go to the Common LFO and use the Box Matrix to select which and how much of each Element is biased to the Common LFO

E-Level is the parameter you can use to set individual volume levels of each Element (and is offered in addition to "volume" which is common).
E-LFO is the Element LFO each Element has its own LFO which is individually set, you can apply control via a controller..

Any explanation as to why those element switches are disabled, and how to enable them for a new voice created by editing a Preset?

My guess would be that Impromptu was a Pipe organ sound that was carried over from the original Motif (classic) and Motif ES era when the Voice architecture only supported 4 Elements - so there might be some anomalie in how the data impacted the 8 Element engine.

Since it has no significant impact on programming or even in changing the setting, it is just one of those little "Easter Eggs" in the system.
Again, if you want to place a check mark next to an Element - simply change the DESTINATION parameter to one where it is ELEMENT selectable - these will all start with the check marks turning red... (not greyed-out, or missing).

Bad Mister wrote:

Common Destinations are always disabled for the individual Elements. The parameter if selected as a Destination will be applied to ALL ELEMENTS whether the grey-out check mark appears or not.

Originally, it had seemed to me that missing gray check marks meant the control was disabled for that element - thus my opening post. When I was testing for this before, I probably failed to get the controller to "catch" the current setting, so I was misled into thinking it was having no effect. I have since gone back and verified that all elements are indeed controlled as you stated, regardless of the appearance (or not) of a gray check mark for each element.

Even though it doesn't matter functionally, it still bothers me that the gray check marks can be missing, as it makes the Control Set display incomplete and potentially confusing. Fortunately, I stumbled across a way to turn on the gray check marks: Simply go to a Destination which does allow individual-element selection, turn all eight element switches on, then change the Destination back to its previous setting. Voila - all eight gray check marks will show. It's admittedly a bit of work for nothing, but it makes me feel better that the display is telling the truth. 😉