Something similar just happened to me today. I landed in ORD (luckily as the weather in NYC was awful today) to find my return flight ORD-LGA Wed night was cancelled and I was rebooked for Thursday AM at 6am. I thought it was strange to cxl a flight that far in advance even if there will be weather. Called EXP line and the agent was showing my original ORD-LGA Wed evening was still operating and she had no idea why I was booted off that flight even after putting me on hold a few times. She got me back on that Wed evening flight with my UPG too. very strange!