Thanks. I'll elaborate a bit, we're considering converting the forum to new software: Discourse.
For the usernames this forum software can only allow standard English letters (no accented letters like é, ø etc.), numbers and the underscore ( _ ) character. The full name/ real name will remain in the original format.
The 'simplified' name will be used as display name and login name for the user and will be used in new quotes.
The users real, correctly written name will be used on his profile page and can be seen with one click from each post (the black box you see with the big "S" symbol is the pop-up you see when clicking on a username).
Any old quotes will also remain to have the old, full name.