If you edit and replace the odd symbols, it should then be in the correct format afterwards

Essentially it's stored a '?' in the database for some characters, for a set of posts from when the problem started until today, so the post when editing will have an actual '?' in it. If you replace that with the correct symbol it'll be OK from then on as it will also be replaced in the database.
Unfortunately as each '?' could be one of any number of characters, we can't just go replace them in the database on everyone's behalf, it's too late. Happily it was only for a relatively short period in the context of the lifespan of the forum!