I also seem to remember previous instalments being single episodes, rather than series, so if anything, they have actually increased their commitment to the series.
Er, it depends. 35 Up was a single episode, but it was over two hours long (delaying News at Ten) when it was shown on ITV in May 1991. 42 Up (on the Beeb as we know) was two episodes of just over a (BBC) hour, and 49 Up was two episodes, but ninety minutes long (this in the era when the news was at 10.30). So there's probably pretty much the same amount made each time, about three hours, but shown in differing formats.
7 Up 2000 is mentioned on another thread - the Beeb have shown it in 2000, 2007 and 2014, although it's never really caught on as the original did.