I think in the case of something in a public place and quite visible, there is an argument to be made for changing a name if its original meaning has almost entirely fallen out of usage. Then, to your average passerby, the name would only seem offensive. If, as Johnny J says, the phrase is still used in the area and most people who see the sign catch the meaning immediately, it would be silly to change, I think.

Sort of makes me think of a story my high school German teacher told. When she came to the USA as a child, she did a lot of her learning of English from dictionaries and material made in the UK. She asked a classmate to 'pass her a rubber' and people laughed, because here that tends to means condom and not eraser. While that is a much milder example, it is just another way words mean different things place to place. That pub name would be unacceptable where I am, but so would calling a cigarette a fag.

-Teefy