In your case, your display name CAN be changed. I had the same issue and Support allowed me to choose my current, normal display name. Please note that I knew about the possible bugs and issues I could encouter, so to prevent them I decided to not make a character or do anything with the account until after the display name change. Under this condition, I have not had a single bug with it. Everything works fine. But I guess it may be different when you already have characters, unlocks, trading post entries or guild memberships (etc.) linked to the old display name.