Professor Andrei Lankov on Kookmin University in Seoul has written an interesting article on how Korean writing has changed over the years. For many centuries, much of Korean writing was done in classical Chinese. Classical Chinese differs greatly from modern Chinese. While today’s Chinese students are schooled in some classical texts, the simplified characters have made it difficult read older texts.
In Korea, you used to see Chinese characters mixed in extensively with the Korean alphabetic script, much like how Japanese does today. Nowadays, it is rare to see them, except for some very commonly understood ones. Unfortunately, this means that many Koreans are cut off from writings of their ancestors. While one may find letters and diaries of ancestors among a family’s possessions, those ancestor’s descendants often can’t read them. This is also true of Vietnam, where the Latin alphabet has totally supplanted the old character script once used. (Vietnam, unlike Korea, coined a large number of characters used only in that language and used for only pure Vietnamese words.)
You may wish to Professor Lankov’s article on how much writing has changed in Korea.