Single word to describe someone who changes topics a lot
A somewhat broader term that might suit, depending on the circumstances, is flibbertigibbet
A frivolous, flighty, or excessively talkative person.
Oxford Dictionaries Online
Also, the term scattershot could be applied to the discourse (but not the person)
Covering a wide range in a random way; indiscriminate: "his habit of scattershot comment on whatever issue catches his eye" (Howell Raines).
American Heritage Dictionary