Who changed the Fatima prayer? From the Fatima prayer we read where Mary says:


  I want you to continue saying the Rosary every day. And after each one
  of the mysteries, my children, I want you to pray in this way: O my
  Jesus, forgive us our sins, save us from the fire of hell. Take all
  souls to heaven, especially those who are most in need.


Now that part in Rosary has changed to: 


  O my Jesus, forgive us our sins. Save us from the fires of hell. Lead
  all souls into heaven, especially those in most need of thy mercy.
  Amen.


What is the history of the added: of thy mercy. This was also added in other languages not just English.
Who changed the Fatima prayer?

I can vividly recall reading the fact that 
Bishop José Alves Correia da Silva of Fatima (1920- 1957) wrote the first official Fatima Prayer in Latin. Delivered by Our Lady during the Third Apparition of Our Lady of Fatima to the three shepherd children on July 13, 1917. It is traditionally prayed after the 'Glory Be' for each decade of the Rosary. (Will add a source if I can locate one).


  O Jésu mi, ignósce nóbis, libera nos ab ígne inférni, ad caélum tráhe ómnes ánimas, praesáertim máxime indigéntes.  Amen. - Oratio Fatimae


Here is the original in English:


  O my Jesus, forgive us our sins, save us from the fire of hell. Take all souls to heaven, especially those who are most in need. - On July 13, 1917, Our Lady of Fatima Showed a Vision of Hell and Taught Us How to Avoid It


Here is how the June 13 (1917) vision is explained:


  On June 13, 1917, the children slipped away for their second meeting with the Virgin Mary at the Cova da Iria. The feast of St. Anthony fell on that date, and the townspeople were celebrating at the church. To their surprise, a few people who had heard their story had already beaten the children to the spot near the holmoak. The Virgin Mary appeared once again, and told the children, I want you to come here on the thirteenth of next month. I want you to continue saying the Rosary every day. And after each one of the mysteries, my children, I want you to pray in this way: O my Jesus, forgive us our sins, save us from the fire of hell. **Take all souls to heaven, especially those who are most in need**. I want you to learn to read and write, and later I will tell you what else I want of you. The Virgin Mary also revealed to Lucia that Jacinta and Francisco would be taken to heaven soon, but Lucia was to remain on Earth longer. The Virgin Mary told Lucia not to be sad, as She would always be with her. The crowd saw a flash of lightning, and some said they saw a hazy cloud near the tree, but none of them actually saw the Virgin Mary as the children did. Some were convinced that the story was true. Others, such as Father Ferreira, were not convinced. - Fatima - The Apparitions


I do not believe that any individual is actually responsible for the additional words used in the Fatima Prayer. Local devotions, customs and translations simply brought this about.

Some websites still have the original Oratio Fatimae on them, but at the same time have a little addition at the end of it, in the English translation:


  Oratio Fatimae  [The Fatima Prayer]
  
  Domine Iesu, dimitte nobis debita nostra, salva nos ab igne inferiori, perduc in caelum omnes animas, praesertim eas, quae misericordiae tuae maxime indigent.
  
  [O my Jesus, forgive us our sins, save us from the fires of Hell, lead all souls to heaven, especially those in most need of Thy mercy.] - Latin Prayers


Personally, I pray the older Latin version, but I can see the merit in praying the newer version in Latin also. It does not take away from the original prayer but adds to it.


  Oh mi Jesu, dimitte nobis debita nostra, libera nos ab igne inferni, conduc in caelum omnes animas, praesertim illas quae maxime indigent misericordia tua. Amen. - Modern Oratio Fatimae


The following article may help shed some light on this subject:


  The True Fatima Prayer
  
  "O my Jesus, forgive us and save us from the fire of Hell, lead all poor
   souls toward Heaven, especially those who are most in need."
  
  In English this prayer is commonly called, "The Fatima Prayer"; in
   Portuguese, "O meu Jesus" prayer.
  
  The Portuguese Text is sometimes given as:
  
  O meu Jesus, perdoai-nos e livrai-nos do fogo do inferno. Levai as almas
   todas para o Ceu, e socorrei principalmente aquelas que mais precisarem.
  
  But the actual words of Our Lady, according to Sr. Lucia are:
  
  O meu Jesus, perdoai-nos e livrai-nos do fogo do inferno, levai as alminhas todas para o Ceu, principalmente aquelas que mais precisarem".
  
  Even if you have not studied Portuguese, you can examine both versions and
   find the textual differences.
  
  In English we commonly use the translation of the first version of the
   text, cited just above, which is:
  
  O my Jesus, forgive us our sins, save us from the fires of Hell. Lead all
   souls to Heaven, especially those most in need of Thy mercy.
  
  The more accurate version would be the translation of Our Lady's actual
   words, as Sr. Lucia reports them:
  
  "O my Jesus, forgive us and save us from the fire of Hell, lead all poor
   souls toward Heaven, especially those who are most in need."
  
  Literally: "O my Jesus, you pardon us, you exempt us of the fire of the hell; you take little souls all for the Sky, mainly those that more to need. "
  
  I would point out that this English translation is not exactly correct;
  because the Portuguese does not say "souls", but "little souls", a term of
  endearment among Portuguese Catholics for the souls in Purgatory, equivalent
  to our phrase "poor souls". The context of the phrase refers to the deliverance of all souls from purgatory into heaven; and thus never signified universal salvation.


This whole question reminds me of the following questions:

Has the Catholic Church ever attempted a local substitute for the word: Amen?

How to propose a change in a prayer?