FranticFish - 11:43 am on May 7, 2013 (gmt 0)
Interesting thread. I don't work on a huge number of sites, or very large sites, so I lack lots of data of my own, but I made very significant changes to a geographic directory type site recently (pretty much a complete overhaul: new urls, new titles, new meta data, new headers, new content on 40 'category' pages and about 120 'product' pages) and saw only immediate improvements that stuck as soon as they were spidered.
We added new text to the 'category' pages - a few paragraphs.
We added/amalgamated a lot of content into the 'product' pages.
No new links at all.
This was the first time any part of the site had been changed in 4-5 years.
But on other, smaller, sites I have seen wobbles that settle after making smaller changes to only one or two pages.
The only difference I can think of is that the directory-type site has been first for its main (non-geographic) term for years and still is. Other sites I saw wobbles on were on page 1 (but not at the top) or page 2.
If this part of the algo is in use then perhaps:
(a) if the site is trusted enough then it gets a bit of a free pass when making changes?
(b) how often 'SEO' elements like the title tag have changed in the past come into play?
The amount of volatility in the SERP might come into play too.