Related על ביצה 2:2

Rabban Shimon b. Gamliel says: The house of Shammai used to say that a priest because of his Heave no immerse on public holiday bath. But he may go into immerse when he passes from one purpose to another and from one group to another. What does it mean "when he passes over from one purpose to another"? If he wants to use his dough on his press or use his press for his dough, he immerses on the same day.