When I re-watch the series each time I look for the changes in Clegg. One thing I've noticed is that it mostly happened after the arrival of Marina in Series 8. Although its never explained in the show, in her first appearance they make reference to events that were covered by the stage play. It appears that Clegg was permanently changed by that experience. However, there was an episode in the 90s where we discover that Clegg and Marina were stuck on a lift together at one point. an event that was supposed to have happened "years ago." So maybe that is the explanation. Either way, by the time Seymour arrives Clegg is almost completely changed, though on occasion his earlier persona comes out, such as in "When You Take a Good Bite, Yorkshire Tastes Terrible" in Series 9.