Data point: If you use a main cabin companion certificate and there is confirmed space available to use an RUC, you can apply the RUC and still change flights with the RUC staying.

EXAMPLE
Original Itinerary: Oct 15 - Oct 20 JFK-ATX with RUC applied immediately after booking

Changed itinerary: Oct 18 - Oct 21 JFK-ATX with RUC applied at reticketing with phone agent.

Furthermore, the changed itinerary was ~$50 cheaper than the original itinerary resulting in a $51.20 credit to the original payment method (not a Delta credit).