Gemara

Levi and David both claimed that a certain field was of their fathers. Levi brought witnesses that it was his father's, and David brought witnesses that he used the field for the years of Chazakah.

Rabah: Surely, David is telling the truth. If he wanted to lie, he could claim better, that he bought it from Levi, and used it for the years of Chazakah!

Objection (Abaye): 'Why should he lie' does not override witnesses!

David later retracted: Yes, it belonged to your fathers. I bought it from you. I said that it was my fathers', for (since I made a Chazakah,) I am confident of keeping it like one who inherited his fathers' land.

(Ula): One can, and then change his claim.

(Chachamim of Neharde'a): One cannot.

In two cases, Ula admits that he cannot change his claim. One is if he initially said 'it belonged to my fathers, not to your fathers.' The other is if he did not claim in Beis Din, and after he left Beis Din, he returned and claimed;

We suspect that others instructed him how he should claim.

In two cases, Chachamim of Neharde'a admit that he can change his claim. One is if he later says 'it belonged to my fathers, who bought it from your fathers.' The other is if they were discussing the case outside of Beis Din, and he did not make any claim, and when he came to Beis Din, he made a claim.

It is normal that one reveals his claims only in Beis Din.

The Halachah is, one may change his claim.

Rishonim

Rif: The Halachah is, one may change his claim, but only if he gives an excuse why he initially claimed differently.

Rambam (Hilchos To'en 15:6): Levi and David both claimed that a field was of their fathers. Levi brought witnesses that it was his father's, and David brought witnesses of Chazakah, Levi gets the field, for David's Chazakah has no claim. If David retracted and said 'yes, it was your fathers'. I bought it from you. I said that it was my fathers', for I rely on it, and it is mine, like my fathers'', or 'my fathers bought it from your fathers', this is a proper claim. He gave an excuse for his first words; we give the field to him. If he initially claimed 'it belonged to my fathers, and not to your fathers', we do not heed his new claim.

Rosh (3:9): When David saw that he lost through his first claim, he fixed his words. His new claim did not totally contradict his first claim; it fixed and explained his first claim.

Hagahos Ashri: When witnesses contradicted his first claim, he cannot make a new claim that contradicts his first claim. If witnesses did not contradict it, he himself may give a different claim to exempt himself, since his first claim did not obligate himself. Now he admits that he sought to exempt himself falsely. All agree that he may retract in this way. He has no need to lie; he could have persisted with his first claim.

Rosh (10): It is normal that one reveals his claims only in Beis Din, lest his opponent think of what to claim against him. If one admitted in front of witnesses, he cannot say 'I was joking.'.

Tosfos (31a DH Mah): Why is the Migo (why should he lie) against witnesses? The witnesses do not say that Levi's fathers never sold it to David's fathers! We must say that 'it is of my fathers'' connotes that it was always theirs.

Poskim

Shulchan Aruch (CM 146:24): Levi and David both claimed that a field was of their fathers. Levi brought witnesses that it was (Rema - always) his father's, and David brought witnesses of Chazakah. Levi gets the field, for David's Chazakah has no claim.

SMA (58): The Rambam did not say (that it was) always of Levi's fathers. The Rema added for, for the Tur says so; in this case, David cannot say that his fathers bought it from Levi's fathers. The Rema distinguishes when the witnesses did not say 'always.'

Rema: If the witnesses said only that it was of Levi's fathers, but not that they did not sell it, if we can interpret David's words to mean that his fathers bought it, we heed David. If David initially claimed 'it was always my fathers'', he cannot give a new claim.

SMA (60,61): We could interpret David's words to mean that he inherited it from his fathers. 'We heed him' means that we claim for him to explain his words, even if he did not.

Shach (23): The Tur connotes that had he claimed 'I inherited it from my fathers', we would claim for him. What is his source? Tosfos says that 'of my fathers' connotes that it was always theirs' (if not, we would say that they bought it), for Tosfos holds that if a Machazik says that he saw the seller there for one day, it helps as if witnesses say that he was there one day. If one says 'it was my fathers'', surely he means that they lived there for a day, so we can claim for him that his fathers bought it. However, the Tur requires witnesses that his fathers lived there for a day. Do not say that David means that his fathers bought it. If so, Tosfos would not need to say that he means that his fathers lived there for a day! Also, if so, we should say so also for a buyer. If a buyer says 'I bought from Ploni, who bought from you', the Tur (CM 21) requires him to say 'I saw Ploni buy it from you.' We do not say that this is what he means! Tosfos said 'of my fathers' to exclude 'my fathers lived there' (then, we claim for him that they bought it), but all the more so 'I inherited it from my fathers' connotes that they did not buy it!

Rema: If Levi said 'it is my fathers'', and David said 'it is my fathers'', this connotes 'it was not your fathers'', so we do not claim for David to interpret his words, unless he explicitly does so himself. However, if witnesses say that David's fathers dwelled there for a day, even if David does not interpret his words, we claim for him.

Shach (24): Tosfos said that 'it is my fathers'', connotes 'it was not your fathers'', to explain why the Migo is against witnesses. We could say that the witnesses testified that his fathers never lived there! Even if one says 'my fathers always lived here', perhaps they rented it and later bought it.

Nesivos ha'Mishpat (Urim 31): The Tur says that 'it is my fathers'' connotes 'it was not your fathers'', when he repsonds to Levi's claim

Shulchan Aruch (ibid): If David retracted and said 'yes, it was your fathers'. I bought it from you. I said that it was my fathers', for I rely on it, and it is mine, like my fathers'', or 'my fathers bought it from your fathers', this is a proper claim. He gave an excuse for his first words; we give the field to him.

SMA (63): The Shulchan Aruch does not discuss a claim 'my fathers bought it from you', for it would require a proof that his fathers were there for a day, since Levi is sure that he himself did not sell it. Levi is not sure that his fathers did not sell it.

Shach (25): In either case, Levi has a Vadai claim 'it was my fathers'.' David's Chazakah is without a claim unless he proves that his fathers lived there for a day.

Shulchan Aruch (ibid): This is if he says so while still in Beis Din. If he left Beis Din, returned and then claimed, we do not heed him, lest others taught him to claim flasely. If he initially claimed 'it was of my fathers, and not of your fathers', we do not heed his new claim even if he is still in Beis Din. One cannot retract and make a new claim when his first claim was in Beis Din. One can retract from what he claimed outside of Beis Din.

Beis Yosef (DH ba'Meh): One can retract from Stam claims outside of Beis Din. If one admitted in front of witnesses 'I owe 100 Zuz to Ploni', he admitted, and he cannot retract in Beis Din.

SMA (64): Since one reveals his claims only in Beis Din, he can even contradict what he said outside of Beis Din.

Shulchan Aruch (ibid): Some say that if he said 'it belonged to my fathers, and not to your fathers', even outside of Beis Din, in Beis Din he can say 'my fathers bought it from your fathers.' Some disagree.

SMA (65): R. Yonah (in the Tur) considers this an admission that his father did not buy it from Levi's fathers. Admission takes effect even outside of Beis Din. Normally, if one did not say 'you are witnesses against me', he can say 'I was joking.' Here is different, for he claims for his own benefit, and admist this he admitted to his detriment.

Gra (59,60): Tosfos (31a DH u'Modu) was unsure about this. Later, the Shulchan Aruch (80:1) rules like the first opinion.