I have been dying my hair black for 2 years, and would like to dye it medium brown. Must I bleach my hair first??

Get it professionally done, I made the mistake of dying my hair red after black through bleaching, and ended up with completely fried/damaged hair, and the color was not what I wanted anyway. Bleach is a terrible base color. You'll have to get your hair completely stripped. Black is the worst color you could put in your hair, as it's the hardest to get out.
Good luck!