I was reading one of my books today by Dave Lowry and he had a short essay about it. He said it is pretty cut and dry about those starting before being sempai and those after being kohai regardless of time, experience and rank. HOWEVER, he did mention that there are minute details that can change things (though he didn't go into detail about what those are)
Nothing can change sempai-kohai. However, saying Person A is sempai and Person B is kohai doesn't say much at all beyond who started earlier. The relationship between sempai and kohai can be as varied as relationships between any two people. It all depends on the time, place, and occasion.