I made a reward reservation (all United planes) on Friday afternoon, and cancelled it online Saturday AM. The website message from United was that they would redeposit my taxes and fees soon and the miles might take up tp 2 weeks. This AM, I noticed the trip was still active in my account, so I called. Yes, it was still active. I will eventually have to make that trip, so I don't think it will cost me anything to change the dates. But it seems to be prudent to make those cancellations by phone from now on, especially if its a paid ticket where change fees can mount up.