I think a clean slate is needed if they want to keep the series going. The whole idea behind the series is focusing on teenagers around 17. If they tried to continue with the old cast I don't think the dynamic would be there anymore with stories having to move onto work/uni/unemployment etc, assuming they tried to keep it realistic. Focusing on a new set of people lets them regurgitate similar themes from the first series. I guess some people will be put off by the lack of freshness, but it also likely to attract people who were 2/3 years too young when it first aired. I hope it is as good as the first and they create a new 'Chris' character, but would be very surprised if it is.