Oddly I'm having issues with this. Even after I combine and then cancel, the system still seems to remember that the larger credits are made up of 2-3 smaller underlying credits, and it ultimately impacts how many of the larger credits I am able to use when I try to book a new flight.
I assume this is for one passenger? Because each passenger has his own credits.
I suggest rolling only one credit from each PNR into each new dummy reservation. Uncheck the second and third ones to leave them behind.