I learned about it while on the phone with the United representative: I specifically asked, "What happens if we end up modifying our itinerary after my Gold Status expires?" She said, to paraphrase, "That doesn't matter: your status for a flight is determined based upon your status when you book the flight. Because you booked this flight while you were a Gold member, then you will remain Gold for this flight."
I clarified further by asking does this mean that we will be recognized and treated as Gold (premium baggage handling, security lines, etc) when we actually fly in February, and she said "yes."