Emily Grierson is not a character that makes a rapid and drastic change at some point during the novel. Rather her change over the course of the novel is more of a slow, spiraling decline. She becomes more and more reclusive and mysterious after her father's death.
Before her father's death, she wasn't a recluse by her own choosing, but her father exerted a great deal of control over Emily. This was most evidenced by his continual refusal to allow any suitor to marry his daughter. When he died, Emily took back some of that control and exerted it over his dead body. She denied to neighbors that he was dead and in the house for three days. Yeah, gross.
Throughout the novel the reader is told about Emily's annual refusal to pay taxes. It's one way that Faulkner can concretely make Emily stand out as an oddity in the town. At one point in the novel, Emily tells the tax collectors that they should ask Mr. Sartoris about her tax deal. At that point in the book though, Sartoris had been dead for more than a decade. It's further concrete evidence that Faulkner provides his readers that Emily's mental faculties are declining.
In section III of the book, Homer Barron begins to court Emily. He gets her out of the house, and together they go on Sunday carriage rides. The downside to her relationship with Homer is that the people of the town think Emily is dating beneath her status level. Lots of gossip ensues. Toward the end of section III Emily purchases a vile of arsenic, and the townspeople worry that Emily will commit suicide with it. Boy are they wrong.
Emily's increasingly warped view of relationships and control culminates in her poisoning and killing Homer. She locks his dead body upstairs and for all intents and purposes acts as if she is married to him. The townspeople know none of this. Emily is smart enough or selfish enough to know that she needs to keep Homer's body hidden. At the end of the novel, after Emily dies, Homer's body is discovered in the upstairs bedroom with Emily's head indentation in the pillow with some hair.
In a nutshell, Emily went from being a controlled daughter to a controlling necrophiliac. Perhaps not sexually aroused by the dead body, but she became so controlling that her only recourse was to choose to be with someone that she could control completely -- a dead body.