Yellow_Sun - 6:55 pm on Jul 25, 2011 (gmt 0)
From my experience, before Panda it was always a little risky when changing/adjusting old pages, often the rankings would drop and gradually come back over time. After Panda, adjusting old pages have kept my rankings the same or they have gone up.
It's tough to say what all exactly is going on, but recently I have changed/updated lots of page that were 2 years old, untouched, ranking well, and I updated/added content, changed titles and headers, linking, etc., and all the rankings went up quickly which is great.
Again, just my experience.