This is a reward ticket and I have no status with UA. I just checked online and find that there is no award ticket available for the day, at least for non-elite member. If I call, can UA change my chedule to my prefered flights even if no award ticket is available?
Maybe. The rule is a 90-minute change. Did your originating flight change as well? If so, you may be able to do it.
If not, and there's something close, they might be able to help regardless.