Why was Benjy’s name changed in The Sound and the Fury? What does this say about the Compton family?
I think this is a wonderful question. Given that this is completely subjective, take my answer for what it is: subjective.
To begin, let us look at the meaning of both names. Benjy means "Son of my right hand". It descends form the Hebrew (Benjamin). Maury means "moor" or "dark-skinned". It descends from Latin (Maurice).
Now, let us look at the character of Benjy. He is mentally challenged and leads readers through his portion of the text in a way which shows the eventual decline of the Comptons is inevitable (given Benjy's decline juxtaposes the decline of the family).
Now, for the name change. Perhaps the meaning of his birth name created too much of a "black mark" on the family. Given his mental retardation, the family probably did not want anything else "staining" his name or the family's name. One "black mark" was enough.
As for the change to Benjy, perhaps it was because he needed to be attended to at all times- to be positioned at the right hand of the father.
Again, this is simply my interpretation of why the name was changed. Only Faulkner would know for sure.
As for the values of the Compton family, this would state that they value how others see them over how they see themselves.
There was an uncle who was no good named Maurice and who came to a bad end. The name was seen as bad luck.
Uncle Maury, in the first section of the novel, appears as a dissipated bigot who fails to achieve any success.
The mother figure in the novel is rather superstitious and, well, odd, and driven by a powerful "attribution of reality" to her fancies. What she believes for a moment becomes real for life. Thus her bed stay and her sickness. Thus Benjy's name change as well. She did not want Benjy to be like his uncle and saw already that bad luck had come of the name with Benjy's natural difficulties.