Sentence:  Only a radical <WSD>change</WSD> in the nature of the population in the central city would be likely to destroy this preference - and we must now turn our attention to the question of whether such a change , gloomily foreseen by so many urban diagnosticians , is actually upon us . 
Meaning of the word marked with <WSD> tags:  a relational difference between states; especially between states before and after some event