Robert_Charlton - 7:37 am on Nov 16, 2012 (gmt 0)
Making some modification could confuse Google algo so it misses the originality.
To some degree, there has always been a question in this regard, and I'm not sure that Panda really changes anything, except that, with Panda, you might have more motivation to improve your pages.
The basic issue, of whether changing a page would cause its originality to be questioned, has been around since the beginning of scrapers. If the changes evolve from previous versions, are made in a human time frame, and they improve the page, then, in my experience, they're not likely to give you problems. That said, I share some of tedster's hesitations about changing a well-ranking page.
I would not repeatedly revise a page in any event, particularly not in reaction to ranking changes. With regard to that, study this thread if you haven't already...
Google's Rank Modifying Patent for Spam Detection
If you're not making spammy changes, this particular patent shouldn't be a concern, but I do think that constant fiddling with a page can make Google think you're trying to manipulate results.
Panda probably helps, as I think it's easier for a human writer to judge whether a page is genuinely useful than it is for a writer to tell whether the page is, say, over-optimized. It's not likely that you can make a page too useful.