House styles differ on the em dash, and some editors ban it outright. Replace every one of them, with the surrounding spacing handled for you.
The hyphen, the en dash and the em dash are three different characters that a lot of software substitutes automatically. The hyphen joins words. The en dash marks a range: 1914–1918. The em dash — this one — sets off an aside. Word processors and phone keyboards convert a double hyphen into an em dash silently, which is how a document ends up with punctuation the author never typed. If your house style bans the em dash, or you are pasting into a system that renders it as a box, you need to find every one of them, including the ones you did not add.
A comma is the usual substitute, because an em dash is nearly always doing a comma's job with more emphasis. A spaced hyphen keeps the visual pause and works in plain-text contexts like a code comment or a terminal. Deleting it outright and leaving a single space is right when the dash was decorative. Whichever you pick, the spacing around the original dash is absorbed into the replacement, so you do not end up with a stray space before a comma. En dashes are included by default because the same rendering problems affect them; switch that off when you have real numeric ranges to protect.
No. Hyphens in compound words like well-known are left alone. Only the em dash, en dash and horizontal bar characters are matched.
They are absorbed into the replacement, so "word — word" becomes "word, word" rather than "word , word" with a stray space.
Turn off "also replace en dashes". The en dash is the correct character for 1914–1918, and leaving it alone preserves those while still removing em dashes.