Every once in a while, if you’re lucky and eat your oatmeal, you get a chance to see a piece of musical history unfold right in front of you. That happened once to Post blogger Michael Conniff, in Boston 33 years ago when he saw the jazz giant Herbie Hancock swing from free jazz of the most radical sort to the funky thang that became “Watermelon Man” and “Headhunters.” The cause of his ruminations: Hancock’s return to the Jazz Aspen Snowmass June festival tonight under the tent at Rio Grande Park. You never know when jazz history just might unfold again right before your eyes.