I'm actually going to be writing my thesis on this (hopefully its done by next April.) I'm doing preliminary research on it now so I don't have that much. Some work has been done on the change in Uncle Tom from a literary perspective but not an historical perspective.

Basically you had the competing images of Uncle Tom vs Sambo and later Uncle Remus. When Uncle Tom's Cabin came out, there was a large Southern literary backlash. Southerners portrayed slaves as childlike and incapable of really doing anything without the supervision of whites.

You also had the minstrel shows of the late 1800s that portrayed Tom as an old man, rather than a young strong man who died to protect runaway slaves.

So basically the idea of the subservient, joyful, childlike slave, or the old slave who did whatever his master wanted, came to be associated with Uncle Tom to the point that it became an insult.

Check back in ~7-9 years and hopefully I'll have published a book on it