This question was bugging me today. I was thinking back to the late 1970s-early 1980s here in my hometown of Buffalo, New York. At that time, Buffalo started the process of replacing mercury and incandescent streetlights to HPS. Now, here's the kick. Many of the incandescents had already been replaced with MV in the late 1960s-early 1970s on side streets with wood poles, but many of the main thoroughfares had either 6.6A Series Incandescent, Mercury, or were already 120 Volt Multiple Mercury circuits. Now to the question. Multiple luminaire replacement (MV to HPS) is a simple matter of replacing the entire fixture, lamp, and P-cell. But what of Series Circuits? For example, Seneca St. had 6.6A MV lights until around 1979. During the changeout, half of the lights were brand new (at that time) General Electric M400 split doors with 250 watt HPS, and the other half Westinghouse OV20s with pole mounted Series Ballasts. How did they manage to keep both sides lit even though it was two totally different types of current?