Not entirely true. I just changed the time for my wife's reward flight to SFO and there was no charge/no problem with United. 'Course, I gave up my SE status with AC and have had 1K UGS status with UA for a couple of years now.

My own policy is to avoid AC skin at all costs, so obviously I would recommend UA. In fact, if you can drive to an American airport, you can also avoid the gross $$$$ that are added to your "reward" ticket by the various charges AC and various levels of governments load on. Heck, my wife's reward ticket cost me $5 and we fly out of Seattle. How much did YOUR last reward ticket cost?