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.