Bava Batra 31 - Chazakah, Case Law # 2 - Amended Pleadings
Two litigants were disputing ownership of a property. One said, "It was the land of my forefathers," and produced witnesses to that effect. The other one produced witnesses that he used the land for three years, needed to establish chazakah.
Rabbah said, "Why should he lie?" The current occupant could have kept the land claiming that he bought it, so we should believe him when he says that he inherited it. But Abaye said that "Why should he lie?" can't be used to contradict witnesses.
The current occupant then changed his claim, saying "Yes, the land indeed belonged to your forefathers, but I bought it from them, and I feel as secure as if it were my forefathers'" and this claim was accepted.
The general rule is: one cannot enter a claim that completely contradicts his previous claim, but he can add information or amend the previous claim by reinterpreting it.