What happened to the receiver to be who was selected before Jonas?
Short Answer: The Receiver who preceded Jonas was released.
In Chapter 18 Jonas asks the Giver about this Receiver and is told that her name was Rosemary and that the Giver loved her as he does Jonas. He tells Jonas that her training went well until he had to transfer the experience of pain to her, particularly the pain of loneliness and loss. The Giver explains,
"I transferred a memory of a child taken from its parents. That was the first one. She appeared stunned at its end."
After this experience, the Giver gave the Receiver only pleasurable experiences; however, she was markedly altered by her experience of pain. "I could see it in her eyes," he tells Jonas. When, in spite of her agony, she insisted that the Giver continue her education, he imparted to her, not physical pain, but different types of anguish, such as poverty, hunger, and terror. Finally, after the finish of one day, Rosemary kissed the Giver's cheek and went to the Chief Elder, requesting release. "I never saw her again," the Giver adds.
After Rosemary was released, the memories which she held went into the community and they were "overwhelmed" by so many feelings since they had never experienced such depth of feelings before. And, then, the memories were lost forever. Having heard all this, Jonas realizes what a tremendous burden it is that he will bear as he must "lift the burden" for all the others.
In Chapter 18, the Giver explains to Jonas what happened to the previous Receiver of Memory. The Giver begins by telling Jonas that the previous Receiver of Memory was named Rosemary and mentions that he loved her. He proceeds to tell Jonas that initially Rosemary was enthusiastic about receiving the memories. However, the Giver mentions that it broke his heart to transfer painful memories to Rosemary. For the first five weeks of Rosemary's training, the Giver only transferred delightful memories to her before giving her painful memories of loneliness and loss because he could not bring himself to share physically painful memories with her. The Giver tells Jonas that after he gave Rosemary the difficult memory of a child being taken away from her family, everything changed. Rosemary fell into a severe depression and eventually requested to be released. When Rosemary was released, her painful memories were loose throughout the community. The Giver mentions that the citizens had a difficult, chaotic time dealing with Rosemary's uncomfortable, painful loose memories, which is why Jonas cannot request to be released.