As long as your are changing to a flight on the same date its not a problem. For example, you would not be able to change from an 11PM flight to the 1245AM flight because the date changed. But in your scenario its not a problem. You really should consider the risk factor as stated in a previous post. Another airline could cancel, or earlier flights could be oversold and passenger moved to the flight you want to take.
Why not consider looking into whether changing the ticket would only cost you the $50 change fee?