Chaim,

Actually, according to Rashi (DH O Ihu) it seems that this is impossible. At best, one can keep the woman (or man) from marrying altogether (just as through Bechirah a person can decide not to marry altogether, and change his destiny), but one cannot Daven to obtain a different Shiduch than destined.

However, as we wrote in the Insights, other Rishonim learn the Gemara differently and say that the destiny can be changed through prayer. But in either case, the Yerushalmi (Beitzah 5:2) concludes that even if it is changed through prayer, "Lo Kaima", the new marriage will not work out. (The Ritva, though, appears to learn that marital destiny can be changed in a positive manner through amassing merits and Ma'asim Tovim; see Sotah 2a, regarding "Zivug Sheni"; Zivug Sheni might allude to a spouse that has been changed due to Ma'asim Tovim.)

Here is what we wrote about this in our Insights, I hope you find it helpful

-Mordecai

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Copied from Insights to Moed Katan 18:2

ATTEMPTING TO CHANGE ONE'S DESTINED SPOUSE

QUESTION: The Gemara quotes Shmuel who says that a person is permitted to get engaged during Chol ha'Mo'ed. Shmuel says that the reason is because if one tarries and waits until after the festival, perhaps someone else will pray to marry her and will succeed in taking her away. Therefore, one may get engaged during Chol ha'Mo'ed in order to make sure no one else gets her.

The Gemara then gives an example of how a person can take away someone else's intended spouse through prayer. Rava saw a man praying that Hash-m let him marry a certain woman. Rava told him not to pray for such a thing, "because if she is destined to be your wife, so she will be your wife anyway and you do not have to pray for her, while if she is not destined to be your wife, then you are denying Hash-m." Later, Rava heard the person praying that Hash-m should take his life, or take the woman's life [so that he not have to see the woman marry someone else -- Rashi]. Rava scolded him and said, "I told you that you should not pray for such things."

This incident is cited by the Gemara to prove that a person's prayer can change his destiny with regard to whom he marries. However, we see the opposite from this incident! Rava says that one's prayers will not have any effect on whom he marries!

ANSWER:

(a) RASHI says (DH O' Iyhu) that the only way that a person will not be able to marry his intended spouse is if someone else prays that one of them should die. It is not possible to pray that someone else should marry one of the intended partners. This is what Shmuel means when he says, "Shema Yekadmenu Achar" -- not that someone else will marry her, but that someone might pray that she die.

(b) The RITVA explains that after Rava told the person not to pray, the person did not listen and he continued to pray that he be able to marry a certain woman. Indeed, his prayers were answered and he married the woman of his prayers. After he married her, though, the marriage was so difficult that he prayed that either he or she should die.

At that point, Rava said, "I told you that there is nothing to gain by praying to change your destined spouse!" Accordingly, when Rava initially told him not to pray because "if she is not destined to be your wife, then you are being Kofer against Hash-m," he did not mean that it is impossible to change one's destiny; he was just trying to discourage the person, for he saw that the Shiduch would not work. ("You are being Kofer against Hash-m" means that by saying that you want to marry this woman, you are implying that you know better than Hash-m what is good for you.) The only way to pray for a Shiduch in a beneficial way is by doing good deeds and thereby meriting a better Shiduch.

The MAHARSHAM points out that the Yerushalmi (Beitzah 5:2) writes explicitly that even if one changes his marital destiny through prayer, the marriage will not work out ("Lo Kaima"). Also, the Zohar (Mishpatim 109a) says that any children born from such a marriage will be attributed in some metaphysical way to the man who was supposed to be the father of those children, the predestined mate of the woman.

This also seems to be the approach of RASHI on the Rif, and the commentary of TALMID RABEINU YECHIEL.

(c) The NIMUKEI YOSEF and the CHIDUSHEI HA'RAN explain (apparently based on the same Yerushalmi) that if a person changes his destiny through prayer it will not be lasting; eventually the right person will get the right wife, because the wrong spouse will die or divorce her. Their texts of the Gemara reads, "Kafarta Bah" instead of "Kafarta ba'Hashem" -- "if she is not destined to be your wife, then you will be Kofer against her ;" that is, you will eventually realize that you are not fit for each other and you will rebel against her. If so, Rava did not mean that it is impossible to change one's destined spouse, because it is possible to change it temporarily . That is why Shmuel permits one to get engaged on Chol ha'Mo'ed, lest somebody else take her as his wife temporarily.