True but the behaviour of the index generally stays similar as...

True but the behaviour of the index generally stays similar as people keep trading in a similar manner regardless of the changes. The top 200 stocks are still the top 200 stocks regardless of when you look.

Also instead of figuring out how the change might effect the index, it's probably better to look at how peoples perception of the change will effect the index. Things only change because perception changes. Fundamental change takes time in the market generally. Hence the argument in market efficiency... but that is another discussion.