Changing a name due to illness:
It is customary to change the name of a sick person, as the changing of a name tears the evil decree against him. [The name of a person is his channel of life and when the name is changed it has ability to bring the person new life. It is customary to simply add a name to his previous name, and not to completely replace the original name. The new name is to be used by people from now on.]
How is it done? The ill person is blessed in Shul [during a Mi Shebeirach for Cholim] using his new name. [This is customarily done by giving the sick person, or a relative of the sick person, an Aliyah to the Torah, and then recite for him a Mi Shebeirach using the new name. Some are accustomed to recite certain prayers at this time.]
Which name is the main name? The added name is the main name of the person when it is added due to illness. This applies even if the person is still customarily called by his original name. This however only applies if he is also sometimes called by his new name [or uses it to sign documents or get an Aliyah], if however, he is never called by his new name [and does not use it for documents or Aliyah] then it is not considered his name at all.
Which name comes first: One is to mention both the new and old name. The added name, being that it is the main name, is to be written first in all documents. It is likewise to be used first when calling the person for an Aliyah to the Torah. This applies even if the person is still customarily called by his original name, so long as he is at times also called by his new name, as stated above.
What name to add: It is customary to add a name that has a connotation of healing and cure, such as Chaim, or Refael or Shalom, Azriel, and the like. By a woman, some say one is to add the name Chana, Sarah or Yocheved, although not the name Rachel, Leah, Bat Sheva or Tamar. Some write
Must the children adapt the new name for their father: When the father is alive, the new name is to be used, having it precede the original name. If the father passed away due to the illness, then if father was customarily called by the new name, or used the new name for documents or an Aliyah, then the children are to adapt the new name for all legal matters. [However, in such a case his original name comes first. ] If however, the father passed away prior to any of the above occurring then only the old name is to be used for all legal matters.
 Rama Y.D. 335/10; Michaber E.H. 129/18; Rosh Hashanah 16b “Four matters tear the evil decree against a person and one of them is changing the name”; See Yuma 83b
 The source: So was seen regarding Sarah, that after her name was changed she was able to have children and the original decree of barrenness was removed. [Rosh Hashanah ibid]
Selling the child: See Sefer Chassidim 245 that it is a Segula for the parents to sell the sick child to another person, and through doing so the child has his decree removed.
 Shelah Hakadosh Hakdama Beis Achron; Tanya Shaar Hayichud Vaemuna 1; Likkutei Torah Bahar p. 41; Igros Kodesh 5/123; See Tamei Haminhagim p. 105 in name of Yehiel Michel
 See Taamei Haminhagim p. 105
 See Igros Kodesh 13/250 [printed in Shulchan Menachem 5/165]
 Rama Y.D. 335/10; Shulchan Menachem 5/164
 Igros Kodesh 13/250 [printed in Shulchan Menachem 5/165] See however Sefer Hamamrim 1949 p. 90 that when the son of the Alter Rebbe, Avraham, was sick, they changed his name to Chaim Avraham in the presence of a Minyan of Jews, not during Davening or Kerias Hatorah.
 Michaber E.H. 129/18; Mordechai; Hagahos Maimanis 3
 Beis Shmuel 129/35
 Beis Shmuel 129/35
 Michaber E.H. 129/18; Igros Kodesh 13/250 [printed in Shulchan Menachem 5/165]
 See Beis Shmuel 129/34
 Ziv Hasheimos 28/3
 Chida in Devash Lefi Mareches Shin 14, brought in Tamei Haminhagim p. 105
 Sefer Chassidim 244
 Kitzur Shelah Inyanei Sefer Torah, brought in Tamei Haminhagim p. 105
 Kitzur Shelah Inyanei Sefer Torah, brought in Tamei Haminhagim p. 105; Minchas Elazar 4/27; Darkei Chaim Veshalom 929
 Beis Shmuel 129/35