It would need to be done by somebody that knew and loved the series and had the money behind them to do it properly. Any one that took it on simply as a way to make money would turn it into puerile muck in moments. I'm looking at you SyFy.
A few episodes, no more. But I think in any series that was 10 years of less the aging would be gradual and most people would not think about the change in a run of that length.You ever watched The Highlander? The actors visibly age during the course of the series, and it was only 6 seasons. Also, the plot and cast expands and becomes too big for the movies.