I believe they are sticking with its original date-setting, hairdos and all, from the look of the promotional video clips. If I recall correctly, Neil did say he had revised one small bit that he felt was insensitive and that Terry would have agreed to change.
Much of the nostalgia of the book was nostalgic then too. (The original title concept was Just William and the Devil, or something like that, but the people who own the Just William copyright wouldn't allow it. Since as far as I know the Just William books never crossed the pond, possibly most of the US audience won't get that part anyway, nor the Enid Blyton either. It doesn't matter - the book stands on its own. The main issues are still up to date, unfortunately.