Had an interesting/annoying situation mid-itinerary last week. Had flown DCA-JFK-HKG-KUL and spent the night in the transit hotel before intending to take MH for KUL-DPS. Well, MH decided they just wanted to not fly to DPS for a couple of days and cancelled all flights on the day I was flying. (The airport and air space were open. VA, QF, and JQ cancelled all flights to DPS, but MH was the only Asian carrier to do so.) Even better, MH announced in the lounge that they were just canceling tickets and not reacommodating pax on later flights, since they didn't know when they would resume flying to DPS. Knowing how great US's IT systems are (and since USairways.com was still showing my flight as operating normally), I refused to leave the lounge until MH could get someone at US to ensure that my return sectors would not be cancelled for no-showing my non-existent KUL-DPS flight.
MH lounge supervisor spent two hours on hold trying to get someone at a US call center. (I think in the end he was in London.) Apparently he couldn't understand what she was trying to tell him, so I got put on the phone. He was pretty adamant at first that I had to fly on whatever flight MH rebooked me onto to DPS or my return would be voided. Finally, however, I got him to understand that MH was refusing to carry me to DPS. (I'd already booked one of the last three seats on an AirAsia flight leaving 12 hours later than my MH flight.) He got a supervisor to approve voiding the KUL-DPS coupon and reissuing the ticket with my HKG-BOS-DCA flights intact. He also told me no further changes would be permitted, but I figured as much since barring IROPS, there shouldn't have been any changes made after flying from DCA.
Just thought I'd provide a data point on how much fun it can be if things go pear-shaped while you're traveling on one of the remaining 037 USDM tickets.