If a litigant declared to his opponent, "My father is acceptable to me to act as a judge," or "Your father is acceptable to me to act as a judge," or "Three cattle herders are acceptable to me to act as judges," but afterward changes his mind and wants to retract, then Rabbi Meir says that indeed he may retract. Still, the Sages say that he may not.
If he wants to retract before the verdict is reached, all agree that he may. The argument is only after the verdict is reached, and the law in this case follows the Sages - that he may not retract.
Art: Portrait of the Artist by Pietro Ligari