I see that some Muslims think there is something insulting about writing Koran instead of Qur’ân. That just shows that they are so traumatized by Islamophobia that they will see insults anywhere. Actually the only correct spelling is qaf, ra, alif with madda, nun, but my computer can't do that.
Koran has been the English spelling for hundreds of years, just as Moscow is our spelling of Moskva and Algeria is our spelling of al-Jazair. Then along came the Arabists and invented a system of transliterstion using lots of consonants with dots under them or vowels with accents over them. Unless you know Arabic this does not get you any closer to the correct pronunciation.
Qur’ân is actually misleading for English speakers, who do not know the difference between qaf and kaf, and who will not understand why there is an apostrophe in the middle. In English qu is pronounced kw, so people are likely to say quoran, as if it had something to do with quorum. Koran will actually take them closer to the Arabic pronunciation. No insult there!