houdl they change Belfast City to Belgrade City in I'll tell my ma if they are from Belgrade?

Bad example. That song wasn't a Belfast song; that is only the most widely known version of it. My mother knew a version from west of Manchester (UK) from her childhood in the 1920s.

So in that case, no problem changing it to anything that can be made to scan. And as, I argued, by the same token no problem with changes to any other song either.