Umpire ruled on a ball or strike

Question
Once an umpire rules on a pitch (whether its a ball or a strike) can it be appealed to the field umpire? I was under the impression that once its called it can not be changed. The only way it can be appealed is if he has not ruled on the pitch yet.

Answer
If a batter checks her swing, and the umpire calls a ball, the catcher can ask the umpire to check with the base umpire to confirm that the batter did not commit to the pitch.  In this case the call becomes what the base umpire calls.

If the home plate umpire ruled that the check swing was a strike, this can not be appealed.

No other ball/strike calls can be appealed.

Tom