Welcome to FlyerTalk! I'm sorry that you had this negative experience, but I can confirm that the reason is that you made a flight change. The United website, when you change one leg of a trip, will re-fare the other leg to the lowest available fare in your travel cabin. It's intended to be a benefit to the user, as most people would appreciate saving some money, but it does have this side-effect. When changing a refundable ticket before travel has begun, either call and tell the agent you want to retain refundability, or just refund and rebook if the price hasn't gone up.