Thanks to this post on Patrick D's Training Journal I've been able to bring back the expandable post, without influencing older post. I can use it when it is needed, but can leave it out, just as easy :Updated 11:42
It takes a little more time because you always have to edit a post, but that's a little effort for a good result. The only glitch I've so far discovered, is that the "read the full article" also appears at the bottom when displayed on a individual page. It should disappear, or even better, link back to the home page
Updated: Patrick mailed me with an new adress to a blog with an even better code by Ramani from Hackosphere, which I've implemented. Go to http://hackosphere.blogspot.com/2006/11/selective-expandable-posts.html
Now the link disappears when the post is expanded!