My old dictionary has only one listing for holocaust, as a common noun, lower-case "h". New dictionaries also list Holocaust, proper noun, upper case "H". What's the story behind this change?

It's just because the word holocaust in the common noun sense means mass genocide, but after WWII people refer to the genocide of the Jews during that time as The Holocaust, which makes it a proper noun referring to that specific event. I'm surprised that your dictionary doesn't still have a lower case common noun listing as well, though.