Amid the aforementioned discussion someone asked when the shift in English from "Corea" to "Korea" occurred. This is an interesting question and one way to answer this is to look at the titles of books about Korea that were published in the late 1800s and early 1900s. Luckily, Brother Anthony's website makes this easy enough to do. He has a number of posts on references to Korea in maps and books in the 16th and 17th centuries, 18th and early 19th centuries, and in the late 19th century. Looking through those the earliest book title with "Korea" in it was in 1848. The latter link has, at the bottom of the page, a list of books published about Korea between 1870 and 1910, and there we see a number of books with "Corea," but "Corea" disappears from book titles in 1895, at least on that list. The list at the bottom of this page, where Brother Anthony has posted links to his work on older works about Korea, includes more such books and journal articles; there we can find a language guide in 1902 and two later books by missionaries which use "Corea," but by that point the tide had turned. After 1895 the use of "Corea" in book titles became an exception to the rule of "Korea." This, of course, is 9 years before the occupation of Korea by Japan that began at the opening of the Russo-Japanese War, and can hardly be ascribed to them. Still, it would be interesting to look through diplomatic correspondence to see how the various Western Legations referred to Korea. At any rate, if 1848 was indeed the first use of "Korea" in English (in a book about Edward Belcher's 1845 voyage, which I mentioned here), then there was essentially a 50-year period during which "Corea" gradually gave way to "Korea."
"Group of Koreans," from Edward Belcher's 1848 book about his voyage.