When a Person Changes His Name, What Do You Call His Child?

 When a person changes their name, how does that affect the child's patronym or matronym. 

I don't think the question is that important when we call someone for an aliyah, or when we write a kesuva or gett, In those cases, what matters is clarity of identity (what they call 'disambiguation',) so what matters is the name by which the person is known.  I am more curious about the affect when you want to use the "true name," such as for the purpose of a bracha (or the opposite,) and for tefilla.

For example: Bamidbar 21:1, Rashi,

ישב הנגב. זֶה עֲמָלֵק, שֶׁנֶּאֱמַר "עֲמָלֵק יוֹשֵׁב בְּאֶרֶץ הַנֶּגֶב" (במדבר י"ג), וְשִׁנָּה אֶת לְשׁוֹנוֹ לְדַבֵּר בִּלְשׁוֹן כְּנַעַן, כְּדֵי שֶׁיִּהְיוּ יִשְׂרָאֵל מִתְפַּלְּלִים לְהַקָּבָּ"ה לָתֵת כְּנַעֲנִים בְּיָדָם, וְהֵם אֵינָן כְּנַעֲנִים, רָאוּ יִשְׂרָאֵל לְבוּשֵׁיהֶם כִּלְבוּשֵׁי עֲמָלֵקִים וּלְשׁוֹנָם לְשׁוֹן כְּנַעַן, אָמְרוּ נִתְפַּלֵּל סְתָם, שֶׁנֶּאֱמַר "אִם נָתֹן תִּתֵּן אֶת הָעָם הַזֶּה בְּיָדִי"

Evidently, if you incorrectly identify the person about whom you are davening, this weakens or nullifies the tefilla. So I think it's important to know what the person's "real" name is, and you can't just rely on "the Ribono shel olam knows who you're talking about."

This has relevance when a man or a woman changes their name. When we daven for aliyas neshama, or you give a bracha, (and certainly for the opposite of a bracha,) we identify by the father, ben Ploni, the patronym. When we daven for refu'ah, we identify by the mother, ben Plonis, the matronymic. See Panim Yafos end of Be'ha'aloscha regarding Aharon's tefilla for Miriam.

 אמר מרחם אמו שכבר אמרו חז"ל (שבת סו, ב) שכל הלחשים והתפילות וקיום בריאת הגוף, הוא בשם אמו ותפילות עליות נשמה בשם אביו

And the Zohar Lech Lecha p. 84 that sees in the passuk in Tehillim 86:16 וְהוֹשִׁיעָה לְבֶן אֲמָתֶךָ that tefilla for rachamim should mention the mother.  

וְהוֹשִׁיעָה לְבֶן אֲמָתֶךָ, וְכִי לָא הֲוָה בְּרֵיהּ דְּיִשַׁי אִיהוּ עַד דְּאִיהוּ אָמַר בִּשְׁמָא דְאִמֵּיהּ וְלָא בִּשְׁמָא דְּאֲבוֹי. אֶלָא הָא אוֹקִימְנָא דְּכַד יֵיתֵי בַּר נָשׁ לְקַבֵּל מִלָּה עִלָּאָה לְאִדְכָּרָא, בָּעֵי לְמֵהַךְ בְּמִלָּה דְּאִיהוּ וַדַּאי. וְעַל דָּא אַדְכַּר לְאִמֵּיהּ וְלָא לְאֲבוֹי. וְתוּ הָא תָּנִינָן (ד"א ל"ג דהא) דְּדָא מֶלֶךְ כִּדְקָאֲמָרָן.

But other tefillos, we go by the father. So what do you do if the mother or father have changed their names? You were born from Reuven and Sarah, not Sholom Reuven and Chavah Sarah.

This actually came up for me, because my mother in law, עליה השלום, was (really Shelia but her Hebrew name was) Sarah, and when she became ill, she did a shinui hashem to Chava Sara. She lived a decade after that, so the name stuck. When I daven for her children, do I say ben/bas Chava Sarah, or ben/bas Sarah?

I thought it was pashut you go by the name the parent had when the child was born.  The person is X child of Y. The fact that Y decided she is now XY, that's because they are NOT the person they were in the past, and they have a new mazal and a new fate. If they are a new person, well, then, the child that was born to Sarah was born to a different person. 

I asked my father in law, HaRav Sholom Reuven Feinstein, and he said you go by the new name.

I later saw that in Orchos Rabbeinu I:64 he quotes the Chazon Ish as saying you can use an adoptive mother's name for tefillah. This shows that the Chazon Ish held that mentioning the mother or father is not magic, it is an invocation of the quality and character of a relationship. He apparently held that what mattered was the loving relationship and rachamim between mother and daughter, and that is absolutely present in adoptive parenthood, as I learned from Reb Micha Berger- an adoptive mother is a true mother. The new name obviously does not change the emotional relationship; the person with the new name is the mother, the man with the new name is the father, and tefillos should mention them as they are known now.  This supports my shver's position.

I was thinking about this because I came across something in Taama d'Kra in Korach, Bamidbar 16:1, as follows:

ואון בן פלת. הוא הי' דודם של דתן ואבירם לפמש"כ בחזקוני ובבעלי התוס' החדש פ' ויגש דפלת הוא פלוא בן ראובן, וי"ל דאחר שעשה תשובה שינו את שמו לאון בן פלת ע"י שנעשו לו פלאות כמ"ש בסנהדרין ק"ט ב' וי"ל דלכן כתיב בפ' פנחס ובני פלוא אליאב ובני ל' רבים (עי' ב"ב קמ"ג ב') משום שהי' לו עוד בן ולא חשיב לי' בהדייהו משום ששינו אצלו שמו של פלוא ועי' בפ' פנחס כתבנו באופן אחר.

He is saying that because Ohn's name was changed from the original and true ben Falu to become a descriptive name ben Peles, when we are listing the children of Falu we can only specify Eliav ben Falu, but not Ohn, because he was now known as ben Peles. This is even though all it says in Pinchas is "the sons of Falu were Chetzron," and it could have said "and Ohn." It seems that to be identified as a "son of Falu," you have to be officially known as ben Falu, not ben Peles. If you experience a life changing event such that you take or are given a new name, you are no longer the person you previously were. 

As the Ruach Chaim says in the beginning of Avos, שם relates to נשמה. 

 הנה ידוע מה שאמרו רז"ל (ברכות דף ז ע"ב) לכו חזו מיפעלות ה' אישר שה שמות בארץ, אל תקרי שמות אלא שמות. כי באמת השם הוא עצם הנפש של האדם, והראיה מדכתיב וייקר א האדם שמות, וכתיב (בראשית ב,יט) וכל אשר יקרא לו האדם נפש חיה הוא שמו

If anyone has heard something about this, please let me know.