This may technically be a bit of a liminal situation between pre-2/1 and post-2/1...I was on hold for hours last night as a DM, tried the various chat features, and everything took forever. Wait time on the phone kept going UP, after the online chat feature reached the 60+min quoted time, it then said an agent would be with me in about 85 more minutes, and the texting feature had an agent (after a 45 minute wait) saying she needed to check with the medallion desk, followed by radio silence.
After explaining the circumstances, an agent this morning let me apply my GUC from Economy to Delta One on JFK-ATL-JNB. So, success! Now to find an agent who knows how to apply them to a KLM-operated flight for the return.